What is the evolutionary advantage of red-green color blindness?

What is the evolutionary advantage of red-green color blindness?

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Red-green colorblindness seems to make it harder for a hunter-gatherer to see whether a fruit is ripe and thus worth picking.

Is there a reason why selection hasn't completely removed red-green color blindness? Are there circumstances where this trait provides an evolutionary benefit?

Short answer
Color-blind subjects are better at detecting color-camouflaged objects. This may give color blinds an advantage in terms of spotting hidden dangers (predators) or finding camouflaged foods.

There are two types of red-green blindness: protanopia (red-blind) and deuteranopia (green-blind), i.e., these people miss one type of cone, namely the (red L cone or the green M cone).

These conditions should be set apart from the condition where there are mutations in the L cones shifting their sensitivity to the green cone spectrum (deuteranomaly) or vice versa (protanomaly).

Since you are talking color-"blindness", as opposed to reduced sensitivity to red or green, I reckon you are asking about true dichromats, i.e., protanopes and deuteranopes. It's an excellent question as to why 2% of the men have either one condition, given that:

Protanopes are more likely to confuse:-

  1. Black with many shades of red
  2. Dark brown with dark green, dark orange and dark red
  3. Some blues with some reds, purples and dark pinks
  4. Mid-greens with some oranges

Deuteranopes are more likely to confuse:-

  1. Mid-reds with mid-greens
  2. Blue-greens with grey and mid-pinks
  3. Bright greens with yellows
  4. Pale pinks with light grey
  5. Mid-reds with mid-brown
  6. Light blues with lilac

There are reports on the benefits of being red-green color blind under certain specific conditions. For example, Morgan et al. (1992) report that the identification of a target area with a different texture or orientation pattern was performed better by dichromats when the surfaces were painted with irrelevant colors. In other words, when color is simply a distractor and confuses the subject to focus on the task (i.e., texture or orientation discrimination), the lack of red-green color vision can actually be beneficial. This in turn could be interpreted as dichromatic vision being beneficial over trichromatic vision to detect color-camouflaged objects.

Reports on improved foraging of dichromats under low-lighting are debated, but cannot be excluded. The better camouflage-breaking performance of dichromats is, however, an established phenomenon (Cain et al., 2010).

During the Second World War it was suggested that color-deficient observers could often penetrate camouflage that deceived the normal observer. The idea has been a recurrent one, both with respect to military camouflage and with respect to the camouflage of the natural world (reviewed in Morgan et al. (1992)

Outlines, rather than colors, are responsible for pattern recognition. In the military, colorblind snipers and spotters are highly valued for these reasons (source: De Paul University). If you sit back far from your screen, look at the normal full-color picture on the left and compare it to the dichromatic picture on the right; the picture on the right appears at higher contrast in trichromats, but dichromats may not see any difference between the two:

Left: full-color image, right: dichromatic image. source: De Paul University

However, I think the dichromat trait is simply not selected against strongly and this would explain its existence more easily than finding reasons it would be selected for (Morgan et al., 1992).

- Cain et al., Biol Lett (2010); 6, 3-38
- Morgan et al., Proc R Soc B (1992); 248: 291-5

There seems to be some evolutionary advantages to red-green colorblindness. The paper in reference 1 (a summary can be found in reference 2) shows that people with red-green color blindness can differentiate between much more shades of khaki than unaffected people. This might help detecting camouflaged food in a green environment.

Reference 2 quotes an expert about this:

For example, it may have helped them spot potential food items in complicated environments such as grass or foliage, he suggests.

This fits with the observation that in a number of new world monkeys dichromatic and trichromatic animals are present in the populations. They found that the dichromatic monkeys have advantages in low light conditions.


  1. Multidimensional scaling reveals a color dimension unique to 'color- deficient' observers
  2. Colour blindness may have hidden advantages
  3. A foraging advantage for dichromatic marmosets (Callithrix geoffroyi) at low light intensity

Already John Dalton wrote about his color vision deficiency. Red, orange, yellow, and green all appeared to be the same color to him. The rest of the color spectrum seemed to be blue, gradually changing to purple. Dalton concluded already in the year 1798, that he can not see long wavelength red light—known as protanopia today.

Some recent genetic analysis of Dalton’s preserved eyes showed, that he was suffering from deuteranopia—another form of red-green color blindness. But anyway this is the first description of the red-green color vision deficiency.

In 1837 August Seebeck carried out some systematic color vision tests and found two different classes of red-green color blindness with differences in severity from weak to strong in both classes.

After that investigations started to gather more details and scientists learned a lot more about our color vision: The genetic source of color vision, its deficiencies and the precise knowledge about the mechanism of color vision in our eyes.

How the Eyes Perceive Color

The eye perceives color with a specific type of photoreceptor cell in the retina called a cone. (Photoreceptors are the cells that detect light rods are the other type of photoreceptor cell.) Cones are concentrated in the center of the retina besides perceiving color these cells make it possible to see fine details.

The retina has approximately 6 million cones. Each type of cone is sensitive to different wavelengths of visible light. There are three types of cone cells, each making up a certain percentage of the total cones in the retina:  

Color blindness can occur when one or more of the cone types do not function properly.  

Is Being Colorblind Actually an Advantage?

Peter Macdiarmid/Getty Images

This story originally appeared on the Conversation and has been reprinted with permission.

The “new world” monkeys of South and Central America range from large muriquis to tiny pygmy marmosets. Some are cute and furry, others bald and bright red, and one even has an extraordinary moustache. Yet, with the exception of owl and howler monkeys, the 130 or so remaining species have one thing in common: A good chunk of the females, and all of the males, are colorblind.

This is quite different from “old world” primates, including us Homo sapiens, who are routinely able to see the world in what we humans imagine as full color. In evolutionary terms, colorblindness sounds like a disadvantage, one which should really have been eliminated by natural selection long ago. So how can we explain a continent of the colorblind monkeys?

I have long wondered what makes primates in the region colorblind and visually diverse, and how evolutionary forces are acting to maintain this variation. We don’t yet know exactly what kept these seemingly disadvantaged monkeys alive and flourishing—but what is becoming clear is that colorblindness is an adaptation not a defect.

The first thing to understand is that what we humans consider “color” is only a small portion of the spectrum. Our “trichromatic” vision is superior to most mammals, who typically share the “dichromatic” vision of new world monkeys and colorblind humans, yet fish, amphibians, reptiles, birds, and even insects are able to see a wider range, even into the UV spectrum. There is a whole world of color out there that humans and our primate cousins are unaware of.

Yet while the eyes of insects and mammals look very different, they work in a remarkably similar way. Both capture and process electromagnetic waves reflected from objects or radiated from luminous sources. Both their eyes contain cells called rods and cones. Rods are specialized for low light levels, providing a sort of night vision. Cones are responsible for color vision, balancing blue, red, and green to provide the perception of the visual spectrum of light. A problem in any cone type causes problems with color perception.

In the most common form of colorblindness, people have difficulty distinguishing red from green. Our colorblind ancestors may have found it tough to recognize when someone was blushing, for instance, or they may have had trouble choosing a ripe fruit or spotting snakes with colorful warning marks.

Similarly, a South American primate might have difficulty identifying social signals such as the bright red head that indicates fitness in the bald-headed uakari. They may find it tough to identify ripe food or colorful threats such as an orange-furred ocelot or jaguar up against a green forest background.

But colorblind vision might actually be an advantage in some situations. After all, color signals can be overwhelming, leading us to pay more attention to colors than patterns. Predators can exploit this by using camouflage to ambush their prey, so the ability to spot a threat is significant.

Colorblind people don’t have this same overload and are often able to see through the deliberate “noise” of colored camouflage to spot the deeper patterns. During World War II, colorblind men were employed to break through camouflaged enemy positions and thereby spot possible targets for bombing. A certain colorblindness may also help create patterns as well as spot them: Vincent Van Gogh was able to create amazingly complex colorful patterns yet his palette shows a striking resemblance to defective color vision.

Low light also negates the advantage of regular eyesight. Even a trichromat won’t see colors in dim light conditions, such as in dusk or dawn, and this is a relatively bigger disadvantage for people—or monkeys—used to seeing the world in “full” color.

The ability to break camouflage and better vision under the dim light are accepted as advantages of a dichromatic color vision. This is backed up by research at Belfast Zoo and in the wild at a research station in Peru’s Amazon rainforest, which found colorblind tamarins were much better than their trichromatic cousins at catching camouflaged crickets that tried to mimic bark or leaves.

However, these factors alone do not explain the maintenance of new world monkeys’ colorblindness. While they explain that there are advantages in being dichromat or trichromat, it does not explain why individuals in the same group share both color vision systems. This polymorphism in primate species is unique among mammals, and clearly there are major advantages still to be discovered.

The more I study this topic, the more I realize how curious primate vision is for example, tetrachromatic people able to see “invisible colors” have recently been discovered. It’s thrilling to imagine which benefits of “defective” color vision are still to be discovered.

Evolution of Color Vision in Primates

It is assumed that at the base of the primate lineage, the ancestral species possessed only the SWS1 and LWS pigments and were dichromats (Hunt et al., 1998). The trichromacy seen in some primate species has been achieved therefore not by the retention of SWS2 or RH2 pigments found in other vertebrate groups but by a duplication of the LWS pigment. This duplication allowed for a mutational drift between the two copies generating two spectrally distinct isoforms maximally sensitive at around 530 nm (M pigment) or 560 nm (L pigment). Interestingly, the three major primate groups, prosimians, New World primates and Old World primates achieved trichromacy via different molecular mechanisms after the New World and Old World landmasses split during the Middle Cretaceous around 65 million years ago (Kious and Tilling, 1994).

Deuteranopia – Red-Green Color Blindness

Deutan color vision deficiencies are by far the most common forms of color blindness. This subtype of red-green color blindness is found in about 6% of the male population, mostly in its mild form deuteranomaly.

Normal and Deuteranopia Color Spectrum

When you have a look at the color spectrum of a deuteranopic person you can see that a variety of colors look different than in a normal color spectrum. Whereas red and green are the main problem colors, there are also for example some gray, purple and a greenish blue-green which can’t be distinguished very well.

The well known term red-green color blindness is actually split into two different subtypes. On one side persons which either lack or have anomalous long wavelength sensitive cones (protan color vision deficiency), which are more responsible for the red part of vision. And on the other side deutan color vision deficiencies, which again are split into two different types:

  1. Dichromats: Deuteranopia (also called green-blind). In this case the medium wavelength sensitive cones (green) are missing at all. A deuteranope can only distinguish 2 to 3 different hues, whereas somebody with normal vision sees 7 different hues.
  2. Anomalous Trichromats: Deuteranomaly (green-weak). This can be everything between almost normal color vision and deuteranopia. The green sensitive cones are not missing in this case, but the peak of sensitivity is moved towards the red sensitive cones.

Below you can see a picture with normal colors on the left side and altered colors on the right side. The picture on the right side shows you how a person affected by deuteranopia would see the scenery (picture taken by Ottmar Liebert, some rights reserverd).

Normal Vision Deuteranopic Vision

In the midst of the last century there were different researches published concerning unilateral deuteranopia. Some persons were found which had trichromatic vision in one eye and dichromatic vision in the other. The eye with dichromatic vision had a color specturm related to a deuteranopia color spectrum. One case of such a one-eyed colorblind is described in the article The Spectral Luminosity Curves for a Dichromatic Eye and a Normal Eye in the Same Person.

The one-eyed color blindness is definitely not the common case, whereas deuteranopia and especially deuteranomaly are the most observed cases of all color vision deficiencies. In 75% of all occurrences of color blindness it is a defect caused by the green sensitive cones. The following list shows the approximative rates of deutan defects in our population:

  1. Deuteranomaly, Male Population: 5%
  2. Deuteranopia, Male Population: 1%
  3. Deuteranomaly, Female Population: 0.35%
  4. Deuteranopia, Female Population: 0.1%

These numbers don’t change much, because deutan color blindness as one form of red-green color blindness is a congenital disease. Red-green color blindness is a sex-linked trait and therefore encoded on the X chromosome. Because women have two X and can overcome the handicap of one, men have only one and are therefore more often affected. This circumstance can also be read in the numbers of the table above. More details about the concrete inheritance pattern can be found at The Biology behind Red-Green Color Blindness.

If you are colorblind there is a big chance that you are red-green colorblind, usually green-weak and male. And specially if you are suffering from deuteranomaly, this condition is not as rare as you might think and you even might find some of your friends who’s also suffering under this color vision deficiency.

Read more about Tritanopia and Protanopia—the other two types of color blindness.

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1 Answer 1

It would seem to be correct. The following is an extract the Nature Journal's archive:

For example, in a building camouflaged with large irregular patches of colour, the actual outline of the building may be lost in the jumble of these patterns. But the colour-blind person may be scarcely conscious of the variegated colours, so that to him the outline of the building may be almost unaffected by the camouflage. In the Ishihara test for colourblindness, certain of the cards actually use this principle a faint blue figure is printed on a background of highly coloured dots of various hues. To the normal observer the blue figure is lost against the background, but the colour-blind person may spot it. Again, in the protanopic and protanomalous type of defect, reds and yellows appear darker than usual, and with certain colouring of building and background this could lead to an enhanced contrast and so give the colour-blind person his advantage.

The following are extracts taken from a BBC article:

The Cambridge team tested this idea by asking deuteranomalous and "colour-normal" individuals to report whether they were able to distinguish between pairs of colours that were theoretically predicted to look different to people with deuteranomalous colour blindness, but the same to those with normal colour vision.

The researchers duly found some colour pairs were only seen to be different by deuteranomalous individuals.

In fact, the researchers found people with deuteranomalous colour blindness gave large difference ratings to pairs of colours which appeared indistinguishable to others.

The researchers, led by Dr John Mollon, said: "The present findings recall reports from the Second World War, which suggested that 'colour blind' observers might be superior in penetrating camouflage."

Females Distinguish Colors Better While Men Excel At Tracking Fast Moving Objects

After having put young adults with normal vision through a battery of tests, scientists were able to conclude that females are better at discriminating among colors, while males excel at tracking fast-moving objects and discerning detail from a distance. These evolutionary adaptations might be linked to the hunter-gatherer past of humans.

The scientists published their findings in the journal Biology of Sex Differences (1, 2). Israel Abramov, lead author and psychologist at Brooklyn College, performed the color experiments, finding that men and women tend to ascribe different shades to the same objects.

Males require a slightly longer wavelength than females to experience the same hue. Longer wavelengths are associated with warmer colors, implying that colors like orange might appear redder to a man than a woman. Likewise, green appears a bit yellower to men than women. Men are also less adept at distinguishing among the shades in the center of the color spectrum, like blues, greens and yellows.

Men could detect quick-changing details from afar, and could track thinner, faster-flashing bars within a bank of blinking lights. The team associates this evolutionary advantage down to neuron development in the visual cortex, which is boosted by male hormones. Testosterone means that males are born with 25% more neurons in this brain region than women.

The findings support the hunter-gatherer hypothesis, which states that the sexes evolved distinct psychological abilities to fit their roles in prehistoric society. The advantage would have allowed males to detect predators or prey from afar, and identify as well as categorize these objects more easily.

Female gatherers may have become better adapted at recognizing static objects like wild berries.

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13 Comments on "Females Distinguish Colors Better While Men Excel At Tracking Fast Moving Objects"

Colour blindness is a gift of females to men only in as much as they are carriers alone. They will always have another copy of X chromosome with right set up to be picked up. Their genetic make up for the rods and cones are also finely tuned to get sharp colours with short wavelenghths. Let us hope that they don`t posses eagle`s vision or reptile`s vision to spot ultraviolet light also. Even audition is sharper for ladies just like their voices being shrill. It is all in the genetic make up. After all Eve was born prior to Adam as far as evolution is concerned and they are seniors and longevity is also more. Thank You.

Adam & Eve was a made up story just like most of the Bible – science has proven that already, only stubborn – religious fanatics still believe it. So your comment about Eve has no credibility.

Wow, ignorant much? Their comment about Eve is not even related to the Biblical story about Eve since they say “Eve was born prior to Adam as far as EVOLUTION is concerned.” In the Biblical story, Adam was born first. They’re using Adam and Eve as symbolic figures of the male and female genders and referring to the evolutionary explanation.

I can’t believe I even took the time to explain that to you.

That might have been a bit harsh, but seriously dude…

old news, also, its just a theory, one that is flawed for several reasons, perception of photons into color isn’t simply male to female in comparison, its person to person, the perception is also relative, and thus doesn’t matter, and the hue differences between individuals is minor and an orange apple or a red apple is just as easy to spot, and i have seen some badass female gamers before that gave me a run for my money in things like soul calibur 4 and halo reach, both of which require tracking very fast movements to be good at, and some of these girls were pro, some were horrible at playing mind you, but so were the same ratio of guys (not that statistics have any validity anyway do to variables of location having differently skilled people of different odds), also it could be argued that color ratios would be far more important in hunting than they are in gathering, since camouflaged predators and prey run away and hide, and fruit doesn’t… or rather it could be argued, if evolution were actually a form of design, but its not, evolution is not designed with any purpose, it just happens chaotically and constantly via chemical reactions, and whichever traits happen to survive best, pass on genes, so any theory that suggests any specific purpose for any specific trait is ridiculous as no trait is directly designed for anything, it just happened to do well or not at whatever random task/s and why it survived is impossible to know outside of seeing it, and even then, mostly vestigial traits exist that are often used at later times, also someone may see more shades of one color but less shades of another, cuz colors are perception of photons at different wavelengths, and aren’t tangible objects, so it could be argued that to see color A you must be blind in color B as the wavelengths are different, stating that even if a male to female difference existed, both would be better and worse at seeing colors,just different ones. it could also be argued that human to other species comparison could have the same result. all i know is i have seen color wheels made by female artists, ones showing lines and labels saying what shade is what, and i can see the difference in every color, and i’m male with a full fledged beard sooo… yeah… there goes that theory.

Male humans have 25% more neurons in [the categories germane to this article] than female humans.

Are you arguing that? On what grounds?

_Why_ male and female dimorphism exist in this category is a matter of *schools of thought* debates for the naturalist department. (Sexual dimorphism can be seen systemically –throughout the whole– in most creatures where sexual dimorphism occurred… duh. Eg vertebrates.)

You challenge here the school of thought [“hunter gather”] this article appealed to as a REASON for the dimorphism. Fine. But you are trying to make THAT challenge seem like it challenges the 25% difference itself.

Your technique of challenge saying “[I know dames who do and males who don’t]” (a common standard technique when talking about human gender) belies your tendency to use _anecdote_ and to not get how ‘bell curve’ normalization* works. Ie there are *mode averages* seen even in complicated sets (like a modern _technology bred_ human population). (…Note that the fact there are chihuahua now does not demonstrate much about the morphological development –“evolution”– of mode average** wolves. [**Ie the type naturally selected most often from a litter of variants once.]

Joe is just argueing a very valid point – that yes, in general there may be an advantage, but that doesn’t mean that there are no men who are as good or better than women at color perception – There are plenty of great artists and painters throughout history who happened to be men, and there are also plenty of professional women athletes ( LPGA and WNBA ) who are better than most average men at sports such as golf or basketball. So not every man is better at sports and detecting fast moving objects than every women, and not all women are better than every man at detecting colors or being great designers (there are some men designers that are better than women – I’ve seen plenty of women with bad taste or color perception)

Sorry dear. I am speaking about the pure science which even a high school student of biology knows that sex chromosome related diseases are,1.Thallasemia,(which was predominant in the British Royal Line) among kings, 2.Male Alopacea which denotes predominant male baldness, and 3.Color blindness for which women happen only to pass on to their male progeny.This is purely because male combination of XY sex chromosome, contains only one X and in Y chromosome, the strand is actually withered to practically X being truncated to Y literally, thereby losing some of the copies of genes mentioned above. Hence with only one copy of X holding the said genes, if they are good then it is okay, but otherwise with a defective gene there is no choice for choosing better one unlike women who has two X chromosomes in their XX combination. The exceptions are the fortunate ones to get only the good X chromosome. Moreover, with regard to cones and rods of retina, it should be noted that horse like animals have got only two types of cones Blue and Green so that they are born colour blind. The fish has got only one cone namely blue and it is having only black and white TV which is enough. In human beings all th three cones Red, Blue, and Green operate to get a full collour vision. Red cone doesn`t mean that it will receive the long wave lengths of Red only. It receives all but it is sensitive only to Red wave length. The case is same for other cones. As you have mentioned it is based on adaptation and necessity of evolution only. What is discussed in the article is not that women are more brilliant in sight but the case of frequency of wave lenght of photons in Red region, Green region and Blue region where they are more sensitized for relative higher frequency in the band when compared to men. You should note that they speak in higher frequency in ladies shrill voice which men have lost bu testerone in their adoloscent stage. Similarly their hearing capacity is also a bit more in high frequency region. The frequency gain for women is only marginal from that of men. The same frequency will differ completely for predators to be sensitized by high frequency ultra-violet rays. It magnifies their acuity eventhough they don`t have so many rods in their retina. Women`s genetic make-up is thus very little modified from men by evolution. Mendel`s geneetics talks of only percentage of normal to abnorbal and does not anywhere rule out abnormal ladies or abnormal males. Thank You.

sorry but no on a few things, evolution dictates that normal does not exist, test groups cannot ever reliably represent a species, and male pattern baldness is from excess sebum, fungus and poor nutrition making testosterone convert into estrogen and has nothing to do with male chromosomes, just crappy health, and if i lucked out with a good x chromosome, that reliably, approximately only an estimated half a men are color blind, but truthfully, i don’t know a single guy that cant point out what colors i see, or what any women sees, not trying to argue needlessly, just stating the research has flaws when assuming certain things are true as a base line.

also,many fish see in color, some fish even see in infra red, and many use colorful photophores or scales for attracting mates or attracting prey or showing they are toxic.

Dear Sir, I want to point out two things referring your comments. First of all infra-red is not a colour. Again using colorful photophores or scales doesn`t mean that their vision is tri-coloured. It is only an adaptation for survival and escaping tactics from predators which are all in the gentic make up adaptation for survival. They have the colours on the body but they don`t see them in colour. The colours are seen only in black and white in different shades. Vison is completely a different department. Secondly, male type alolpacea , I mean only the baldnes of the scalp which is obviously predominant among many males and expressing more and more because of reduced selection of faulty X-chromosome and Y lacking in the SRY zone some of the SOX genees missing. Testosterone failure may be true but its production is only genetic. Moreover only growing of moustaches and beards are the department of Testosterone concerned. Little portion of testosterone in females also cause formation of pubic and arm-pit hairs. Scalp hair is completely taking a different route and just depends of sex chromosomes insuffeciency only. By the by nobody will accept that he is colour blind because from the birth he sees the tree leaves as brown only and calls this shade of brown to be green. His acuity in distinguihing colours may be alright but he is unlucky to see in true colours and sees them in only two colour shades and learnt like that. Who said that colour blind males are very few but they are considerably present though not in majority like left handedness. Thank You.

Madanagopal – please see my comment above. Common sense shows that there are exceptions to the rule, that is what we are saying. Not every man is less able to perceive colors than the average women, and not all women are less deficient at perceiving fast moving objects and exceling at sports than the average man.

That is why there are some professionals and geniuses that include men in the fashion & color industry that are more capable of recognizing and distinguishing colors than the average women. Rigid stereotypes, even if supported by science, do not always hold true for some random individuals. That is a fact of reality. So Thank You.

Hello! Mr.anonymous1. By saying that Eve was born before Adam, I am surprised that you take it in the literal sense. You should have a scientific sense and understand that Eve and Adam means women and men only and the name is only symbolic and nothing to do with Bible. This is because X chromosome is longer than Y chromosome and it definitely means that it has withered a part of its strand in evolution and became Y which is the Men related gene. The withering is conspicuous in losing gene responsible for scalp hair, color cone representing gene and Thalasemea or blood clotting gene in men who always suffer with these diseases if their XY combination has got a defective X from their mother who is not herself affected because her other X in XX which will be normal may compensate. Only if both the X of XX are defective which is very rare women will suffer from these diseases. Thus color cone of the retina is also a product of this X gene from women. Thank You.

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Color blindness is an inaccurate term. Most color-blind people can see color, they just don't see the same colors as everyone else.

There have been a number of articles written about how to improve graphs, charts, and other visual aids on computers to better serve color-blind people. That is a worthwhile endeavor, and the people writing them mean well, but I suspect very few of them are color-blind because the advice is often poor and sometimes wrong. The most common variety of color blindness is called red-green color blindness, or deuteranopia, and it affects about 6% of human males. As someone who has moderate deuteranopia, I'd like to explain what living with it is really like.

The answer may surprise you.

I see red and green just fine. Maybe not as fine as you do, but just fine. I get by. I can drive a car and I stop when the light is red and go when the light is green. (Blue and yellow, by the way, I see the same as you. For a tiny fraction of people that is not the case, but that's not the condition I'm writing about.)

If I can see red and green, what then is red-green color blindness?

To answer that, we need to look at the genetics and design of the human vision system. I will only be writing about moderate deuteranopia, because that's what I have and I know what it is: I live with it. Maybe I can help you understand how that impairment—and it is an impairment, however mild—affects the way I see things, especially when people make charts for display on a computer.

There's a lot to go through, but here is a summary. The brain interprets signals from the eye to determine color, but the eye doesn't see colors. There is no red receptor, no green receptor in the eye. The color-sensitive receptors in the eye, called cones, don't work like that. Instead there are several different types of cones with broad but overlapping color response curves, and what the eye delivers to the brain is the difference between the signals from nearby cones with possibly different color response. Colors are what the brain makes from those signals.

There are also monochromatic receptors in the eye, called rods, and lots of them, but we're ignoring them here. They are most important in low light. In bright light it's the color-sensitive cones that dominate.

For most mammals, there are two color response curves for cones in the eye. They are called warm and cool, or yellow and blue. Dogs, for instance, see color, but from a smaller palette than we do. The color responses are determined, in effect, by pigments in front of the light receptors, filters if you will. We have this system in our eyes, but we also have another, and that second one is the central player in this discussion.

We are mammals, primates, and we are members of the branch of primates called Old World monkeys. At some point our ancestors in Africa moved to the trees and started eating the fruit there. The old warm/cool color system is not great at spotting orange or red fruit in a green tree. Evolution solved this problem by duplicating a pigment and mutating it to make a third one. This created three pigments in the monkey eye, and that allowed a new color dimension to arise, creating what we now think of as the red/green color axis. That dimension makes fruit easier to find in the jungle, granting a selective advantage to monkeys, like us, who possess it.

It's not necessary to have this second, red/green color system to survive. Monkeys could find fruit before the new system evolved. So the red/green system favored monkeys who had it, but it wasn't necessary, and evolutionary pressure hasn't yet perfected the system. It's also relatively new, so it's still evolving. As a result, not all humans have equivalent color vision.

The mechanism is a bit sloppy. The mutation is a "stutter" mutation, meaning that the pigment was created by duplicating the original warm pigment's DNA and then repeating some of its codon sequences. The quality of the new pigment—how much the pigment separates spectrally from the old warm pigment—is determined by how well the stutter mutation is preserved. No stutter, you get just the warm/cool dimension, a condition known as dichromacy that affects a small fraction of people, almost exclusively male (and all dogs). Full stutter, you get the normal human vision with yellow/blue and red/green dimensions. Partial stutter, and you get me, moderately red-green color-blind. Degrees of red-green color blindness arise according to how much stutter is in the chromosome.

Those pigments are encoded only on the X chromosome. That means that most males, being XY, get only one copy of the pigment genes, while most females, being XX, get two. If an XY male inherits a bad copy of the X he will be color-blind. An XX female, though, will be much less likely to get two bad copies. But some will get a good one and a bad one, one from the mother and one from the father, giving them four pigments. Such females are called tetrachromatic and have a richer color system than most of us, even than normal trichromats like you.

The key point about the X-residence of the pigment, though, is that men are much likelier than women to be red-green color-blind.

Here is a figure from an article by Denis Baylor in an essay collection called Colour Art & Science, edited by Trevor Lamb and Janine Bourriau, an excellent resource .

The top diagram shows the pigment spectra of a dichromat, what most mammals have. The bottom one shows the normal trichromat human pigment spectra. Note that two of the pigments are the same as in a dichromat, but there is a third, shifted slightly to the red. That is the Old World monkey mutation, making it possible to discriminate red. The diagram in the middle shows the spectra for someone with red-green color blindness. You can see that there are still three pigments, but the difference between the middle and longer-wave (redder) pigment is smaller.

A deuteranope like me can still discriminate red and green, just not as well. Perhaps what I see is a bit like what you see when evening approaches and the color seems to drain from the landscape as the rods begin to take over. Or another analogy might be what happens when you turn the stereo's volume down: You can still hear all the instruments, but they don't stand out as well.

It's worth emphasizing that there is no "red" or "green" or "blue" or "yellow" receptor in the eye. The optical pigments have very broad spectra. It's the difference in the response between two receptors that the vision system turns into color.

In short, I still see red and green, just not as well as you do. But there's another important part of the human visual system that is relevant here, and it has a huge influence on how red-green color blindness affects the clarity of diagrams on slides and such.

It has to do with edge detection. The signals from receptors in the eye are used not only to detect color, but also to detect edges. In fact since color is detected largely by differences of spectral response from nearby receptors, the edges are important because that's where the strongest difference lies. The color of a region, especially a small one, is largely determined at the edges.

Of course, all animals need some form of visual processing that identifies objects, and edge detection is part of that processing in mammals. But the edge detection circuitry is not uniformly deployed. In particular, there is very little high-contrast detection capability for cool colors. You can see this yourself in the following diagram, provided your monitor is set up properly. The small pure blue text on the pure black background is harder to read than even the slightly less saturated blue text, and much harder than the green or red. Make sure the image is no more than about 5cm across to see the effect properly, as the scale of the contrast signal matters:

In this image, the top line is pure computer green, the next is pure computer red, and the bottom is pure computer blue. In between is a sequence leading to ever purer blues towards the bottom. For me, and I believe for everyone, the bottom line is very hard to read.

Here is the same text field as above but with a white background:

Notice that the blue text is now easy to read. That's because it's against white, which includes lots of light and all colors, so it's easy for the eye to build the difference signals and recover the edges. Essentially, it detects a change of color from the white to the blue. Across the boundary the level of blue changes, but so do the levels red and green. When the background is black, however, the eye depends on the blue alone—black has no color, no light to contribute a signal, no red, no green—and that is a challenge for the human eye.

Now here's some fun: double the size of the black-backgrounded image and the blue text becomes disproportionately more readable:

Because the text is bigger, more receptors are involved and there is less dependence on edge detection, making it easier to read the text. As I said above, the scale of the contrast changes matters. If you use your browser to blow up the image further you'll see it becomes even easier to read the blue text.

And that provides a hint about how red-green color blindness looks to people who have it.

For red-green color-blind people, the major effect comes from the fact that edge detection is weaker in the red/green dimension, sort of like blue edge detection is for everyone. Because the pigments are closer together than in a person with regular vision, if the color difference in the red-green dimension is the only signal that an edge is there, it becomes hard to see the edge and therefore hard to see the color.

In other words, the problem you have reading the blue text in the upper diagram is analogous to how much trouble a color-blind person has seeing detail in an image with only a mix of red and green. And the issue isn't between computer red versus computer green, which are quite easy to tell apart as they have very different spectra, but between more natural colors on the red/green dimension, colors that align with the naturally evolved pigments in the cones.

In short, color detection when looking at small things, deciding what color an item is when it's so small that only the color difference signal at the edges can make the determination, is worse for color-blind people. Even though the colors are easy to distinguish for large objects, it's hard when they get small.

In this next diagram I can easily tell that in the top row the left block is greenish and the right block is reddish, but in the bottom row that is a much harder distinction for me to make, and it gets even harder if I look from father away, further shrinking the apparent size of the small boxes. From across the room it's all but impossible, even though the colors of the upper boxes remain easy to identify.

Remember when I said I could see red and green just fine? Well, I can see the colors just fine (more or less). But that is true only when the object is large enough that the color analysis isn't being done only by edge detection . Fields of color are easy, but lines and dots are very hard.

Here's another example. Some devices come with a tiny LED that indicates charging status by changing color: red for low battery, amber for medium, and green for a full charge. I have a lot of trouble discriminating the amber and green lights, but can solve this by holding the light very close to my eye so it occupies a larger part of the visual field. When the light looks bigger, I can tell what color it is.

Another consequence of all this is that I see very little color in the stars. That makes me sad.

Remember this is about color, just color. It's easy to distinguish two items if their colors are close but their intensities, for example, are different. A bright red next to a dull green is easy to spot, even if the same red dulled down to the level of the green would not be. Those squares above are at roughly equal saturations and intensities. If not, it would be easier to tell which is red and which is green.

To return to the reason for writing this article, red/green color blindness affects legibility. The way the human vision system works, and the way it sometimes doesn't work so well, implies there are things to consider when designing an information display that you want to be clearly understood.

First, choose colors that can be easily distinguished. If possible, keep them far apart on the spectrum. If not, differentiate them some other way, such as by intensity or saturation.

Second, use other cues if possible. Color is complex, so if you can add another component to a line on a graph, such as a dashed versus dotted pattern, or even good labeling, that helps a lot.

Third, edge detection is key to comprehension but can be tricky. Avoid difficult situations such as pure blue text on a black background. Avoid tiny text.

Fourth, size matters. Don't use the thinnest possible line. A fatter one might work just as well for the diagram but be much easier to see and to identify by color.

And to introduce one last topic, some people, like me, have old eyes, and old eyes have much more trouble with scattered light and what that does to contrast. Although dark mode is very popular these days, bright text on a black background scatters in a way that makes it hard to read. The letters have halos around them that can be confusing. Black text on a white background works well because the scatter is uniform and doesn't make halos. It's fortunate that paper is white and ink is black, because that works well for all ages.

The most important lesson is to not assume you know how something appears to a color-blind person, or to anyone else for that matter. If possible, ask someone you know who has eyes different from yours to assess your design and make sure it's legible. The world is full of people with vision problems of all kinds. If only the people who used amber LEDs to indicate charge had realized that.