[egypturnash]

So we have four different kinds of light receptors in the eye: three "cones" that pick up a relatively narrow range, with peaks at the wavelengths we call "red", "green", and "blue", and one kind of "rod" that picks up a wider range of wavelengths that tends to kick in more in low light conditions, and gives you more bright/dark sensations than color.

The color we call "yellow" is what you get when the red and green cones are reporting fairly equal amounts of stimulus. It generally corresponds to the wavelengths between them. Similarly, "cyan" or "light blue" is the green and blue cones reporting fairly similar amounts of light. That color is also between the peaks of those cone's responses.

And then there is "magenta" or "purple". Which is what you get when the red and blue cones are reporting light. This does not correspond to any particular wavelength of light. But inside your brain it still produces a sensation of color, that has similarities to the red and blue experiences.

This is why some people like to say that "purple is not a real color": it does not correspond to a single particular wavelength of light. But really "color" is just something your brain makes up to classify different combinations of excitement of the rods and cones in your eyes anyway; the reality is that our eyes are only sensitive to a tiny fragment of the electromagnetic spectrum, with an uneven distribution - the "red" and "green" cones have a lot of overlap in their sensitivity, while "blue" barely overlaps either. Have a look at the diagrams in Wikipedia's page on "trichromacy": https://en.wikipedia.org/wiki/Trichromacy

This is also why all our displays are based on red, green, and blue lights: you can fool the brain into sensing a particular color by showing the eyes nothing but the three frequencies the cones are most sensitive to, in various amounts.