One of the more profound discoveries I’ve made regarding color blindness is that there are only two hues in a color blind person’s world: blue and yellow. For some reason I thought that the hues between blue and green were still vivid colors, like this:

But actually it looks like this (deuteranope simulation):

You can see here that turquoise looks the same as gray/white — in other words, it looks colorless. There are really only two hues: anything between them looks less intense, more gray.

White light is a mixture of all colors – it activates all receptors equally. Because turquoise is right between the two receptors it also activates them equally. The two types of light are providing the same information, it takes a third receptor with a different response to tell the difference. Having a third receptor has a profound effect: all wavelengths become distinct colors, a rainbow of hues is visible. Would adding a fourth receptor have a similarly profound effect? I don’t think so — the spectrum is a linear one-dimensional type of information — but maybe I’m lacking imagination here.

Green traffic lights actually have a bluish tinge to them to distinguish them from the red and yellow lights. Because of this, they actually look white! Here is a digitally merged photo I took, along with a deuteranope simulation:

This colorless white/gray effect for hues that hit both receptors evenly is also visible on the other side of the color wheel, in the “unnatural” hues formed by mixing red and blue.

Thus the colors potentially confused by red-green color blind fellows goes beyond distinguishing between hues in the red-to-green range. Turquoise and magenta can be confused for gray, and purple can look blue. I’ll close here with a series of potential color confusions:

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.