by Rishidev Chaudhuri
I have a child's affection for color, for broad swathes of bright saturated colors, for unapologetic reds and greens and blues and yellows. And yet this strong visceral and emotional reaction often feels immature and undifferentiated. I never really learned the names of the colors properly. This is partly from an artist mother, who would always give me very specific answers when I asked for the name of a color (burnt sienna, cerulean), so that i never quite figured out the broad categories correctly, didn't really remember the specific terms, and came to experience the names of the colors as magical incantations that descended upon sensory impressions according to uncertain principles. And this is partly the result of decreased red-green sensitivity, so that while I can tell pure reds and greens apart easily, and can distinguish expanses of color, I start to stumble at blue with a little bit of red or green added, or at intermediate points along a red-green mixture, or at thin lines of color1. Recently, I've been making graphs that require a large number of data traces on the same figure, and I need each trace to be a sufficiently different color that I can easily tell them apart. And so I've found myself paying more attention to the way colors are described and how to get them on a computer.
It has been known for some time that colors can be described by three numbers. If I show you light of a certain color and ask you to match it by combining lights of three other colors and varying their intensities, you'll typically be able to find a combination that looks indistinguishable. But the wavelengths you combine might be very different from the wavelengths I showed you. Light of the wavelength corresponding to yellow and light of the right combination of red and green wavelengths will look the same, even though they are physically quite different. This structure is reflected in the retina. For the most part, we have three types of color-sensitive cells (cones) and so make three measurements of any color we see, corresponding to light centered around three different wavelengths. Informally, these are said to be peaked around blue, green and red, though the peaks don't quite line up at these colors. Any information that isn't captured in these three numbers is literally invisible. Dogs and cats (and most mammals) measure only two numbers to make a color, rather than three, and seem to see like red-green colorblind people. There is some speculation that a subset of women have cones that make measurements at four frequencies and so differentiate colors that look identical to most people.
There are all sorts of complexities and caveats of course.
