Color Blindness

For some time now, I’ve been wanting to write about red-green color blindness, a dramatic perceptual difference with an interesting genetic and evolutionary story. This first post will mostly be an introduction to the topic. If you are color blind: I always feel guilty when I speak of this as a deficiency, or when I emphasize how profound the differences seem to the rest of us. I hope it doesn’t bother you. I always wish I could pee standing up so… there.

Daylight vision in humans is mediated by the opsin proteins, which transmit signals that activate nerves when they are hit with light. Humans have three different opsins with different sensitivities to the colors of the spectrum — it is the different color sensitivities that allow us to see color. You can call these the “blue”, “green” and “red” opsins.

A normalized diagram of the sensitivities of opsins to different wavelengths of light.

“S” = “short wavelength”, is the “blue” opsin.

“M” = “medium wavelength”, is the “green” opsin.

“L” = “long wavelength”, is the “red” opsin.

In its severe form, red-green color blindness occurs when a man is missing the “green” or “red” opsin – these conditions are respectively known as deuteranopia (1% of all males) and protanopia (another 1% of males). They are fairly similar in effect: a total loss of ability to distinguish hues in the green to red range. There are many less severe forms of color blindness — 6% of males — but that’s a later post.

I say “males” because color blindness is almost always seen in men. This is because the “red” and “green” opsin genes are located on the X chromosome, which men have only one copy of. Women have two X chromosomes; even if one has inherited a deletion mutation, the other can serve as a back-up. For a woman to be color blind, both X’s would have to carry the same mutation, which is much less likely to occur. (e.g. 1% * 1% = 0.01%)

I’ll end this post by showing you what color blindness looks like. Vischeck is a service available online that simulates how images look to a color blind person. To a color blind individual the simulation and original images should look identical (or nearly so – computer monitors vary, so this cannot be perfect). If you’re curious about the algorithm, the program is based on this paper.

Deuteranopia Original Protanopia

All colors in the red to green range — green, yellow, orange, red — are simulated here as yellow. As you can see, deuteranopia and protanopia are almost identical – the main difference is that red looks darker to the protanope (look closely at the picture of cars). Also interesting to note: the butterfly picture demonstrates how purple looks like blue to the color blind individual.

Credits: Opsin sensitivity diagram adapted from Wikipedia diagram, credit goes to User:Vanessaezekowitz and from the screenshot for Wavelength 1.3. Photos taken from flickr users Marshall Flickman, Teo, and Oneras under CC and CC-by-SA licenses.

5 thoughts on “Color Blindness

  1. justin liew

    Well, first of all, FYI, i m color-blind and i would like to say that u actually do not need to feel guilty at all when you speak of color-blindness as a deficiency or deform because the discovery of tetrachromacy in human population indicating that even the “self-proclaimed normal” people (normal trichomacy) are just another group of people with disability (tho they are the majoty in human population currently) compared with the tetrachromats who can see a lot more colors!


    I too, am red-green color deficient. You shouldn’t feel bad about calling it what it is: a deficiency. However, like many other disabilities, it also grants you a few proficiencies in order to compensate:
    We can see better in the dark.
    We can make far more fine distinctions between shades of khaki.
    We have better and faster pattern recognition skills.
    I would also bet that if tested, there would be a high incidence of above-average critical thinking skills.

    Imagine growing up with the ability to see, but a complete inability to trust what you’re seeing as accurate. You’re forced to look for other environmental cues to give you the info you need, and the only way is to apply your mind. You have to outwit the world, and those people in it, if for no other reason than to attain basic functionality within a social framework.

    As for what it’s like, you can look at adjusted photos, or you can try an experiment. Go outside at dusk, just before dark. Then try and tell precisely what color the cars are. You’ll get the major colors right, but all the shades and hues in between look the same grayish color. For instance: To me, the pink of everyone’s mouth is gray. If you tell someone this, they are often unsettled enough to actually cover their mouth. But to me, that’s just the way it has always been.

  3. Madeleine

    That’s a cool way of looking at it. There’s also evidence that colorblind people learn to be more sensitive to texture/pattern differences and can spot camouflaged items more easily (or rather, perhaps, the camouflage is better at tricking people who see green-red because it is designed by people who see green-red) … hopefully I’ll get around to writing more on this…

    I hadn’t realized that about mouths!

    Another thing I realized lately is that laser pointers are horrible (maybe just for protanopes). Maybe blue laser pointers would be good but they’re ridiculously expensive. :-(

  4. R. Hahin

    Dear Madeleine:

    I am a neurophysiologist who studies Na channels and agents that alter them. I also teach a class (Vision & the Visual System) about brain function and vision. I found a diagram of the three spectral absorbance curves for the three cone classes that would be helpful for my students on your site Mad Prime. I would appreciate your permission to use it in my classroom.

    Sincerely yours,
    R. Hahin, Ph.D.


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