eli5: Why is it impossible to imagine new colors?
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Everything you “just know” about color, you learned by having human eyeballs. And human eyeballs turn out to have some weird biases.
Your eyeballs detect color with three kinds of color sensors (cone cells). Each kind responds to different frequencies of light. [But they’re not evenly spaced on the spectrum.](https://en.wikipedia.org/wiki/Cone_cell#/media/File:Cone-fundamentals-with-srgb-spectrum.svg) Two kinds of cone cell (M and L, in the picture) mostly detect red-green light, overlapping with one another, but with slightly different strengths. The other one (S) detects blue and purple.
And which color you *perceive* depends on the *differences* between these cells’ response, in what’s called an “opponent process”. The M and L signals, *taken together,* tell you how red or green something is. The S signal tells you how blue or purple it is. That’s why the most common kind of colorblindness is *red-green* colorblindness. That’s what happens when someone’s M cells or their L cells don’t work.
The color signals from your cone cells, plus the brightness signal from your rod cells, are combined in your brain to make your sense of color vision. Those signals are all your brain has ever learned about what it is like to perceive color. That’s the whole space of color that you can ever see with the kind of eyeballs that you’ve always had.
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