What I always found fascinating which is not addressed here, is how the visible light spectrum is a straight line with other "colors" at its ends, yet somehow our brains cut out that segment of the EM spectrum, and manage to bend it into a circle, so that color "wheels" make sense to us and flow nicely, when in fact red and violet are quite separate on the EM spectrum. It's our brains that play a little trick
I've heard two explanations for this. The first is that the longer wavelength "red"-ish receptors in our eyes are also excited by the frequencies of light at the other end of the visible spectrum, so shorter wavelengths than blue start to appear red again. Unfortunately the graph produced to back this up is usually that of the CIE colour matching functions, which do indeed have an obvious lump around purple, but are not derived from measurements. That lump was added to create a three-primary space which didn't require negative numbers to encode completely pure single-wavelength colours. That's a problem which otherwise crops up when you use three primaries based directly on measurements of the eye, because the three pigments in our eyes are not selective enough that a single wavelength only activates one receptor. Real measurements of the pigments don't really back up the idea of extra sensitivity to purple in the long-wavelength cones.
The second explanation (or suggestion, at least) is more psychological. In the distant past our eyes had only two colour receptors, and our brains measured the relative difference between the two, so we percieved colour on a one-dimensional line - e.g. more "warm" long wavelengths or more "cool" short wavelengths. At some point we evolved a third receptor, and our brains began processing this extra input as another perceptual axis orthogonal to the existing one, which could be described as purple/green against the original red/blue. Exploring this new 2D space results in a continuous rainbow around the edges.
