Why is a color spectrum nicely represented as a circular wheel?
We can idealize the colors that make a rainbow as monochromatic colors–light consisting of a single wavelength. It’s not a perfect model of an actual rainbow, since the light source which generates a real-world rainbow is not a point source, and different wavelengths are not perfectly separated, but it’ll do. When we do this, it turn out that the two ends of the spectrum don’t match up. Take a look at the CIE Chromaticity Diagram. It correlates wavelengths of light, and mixtures thereof, to colors that we actually see. (Note: the colors that appear in the diagram are only approximate, as computer monitors cannot display all colors.) The colors which appear on the curved boundary are monochromatic–consisting of a single wavelength, and appearing in our idealized rainbow. The numbers on the curved boundary indicate the wavelength of such light in nanometers. All other colors in the diagram–both those in the interio
Color theory can get pretty complex pretty quickly, so I’m sure there will be a lot of answers of all sorts here, but here is my take on it (I have some books on color at home and so may elucidate more later): One reason is that color is in large part our perception, and so our brains are wired to percieve colors that way. Another, related fact is that for both subtractive color (like inks) and additive color (like monitors) there are three primary colors that can be combined to produce the intermediate colors; for subtractive color they are the familiar cyan, magenta, and yellow (CMYK, with K being black); for monitors it is red, green, and blue (RGB). In other word, color and the relationships of multiple colors are not determined solely by wavelength but by our perception as well.
If we couldn’t see the light wavelength that our brain currently interprets as “red”, I imagine that our brain’s optic cortex would adjust its color identifier to make orange be red instead. Our eyes have evolved to see the mix of our particular visible wavelengths as “white”. I think if the range of wavelengths were different, we would still see it as “white”, meaning that when split up prismatically, you’d still see the complete color spectrum without any missing pieces. We haven’t evolved to see wavelengths outside of our spectrum because seeing those types of light are not important to our survival. Also, light colors and pigment colors (covered by color wheels) are different beasts. Obviously, if you mix red paint with blue paint (from either end of the color spectrum), you’ll get purple, so the color wheel makes sense because there is a continuum. Meanwhile, in mixing light colors, red and blue will get you magenta. Mixing red and green light will get you yellow.
Cliff Notes version of my comment above, since that ended up being longer than I intended: Some of the purples in the color wheel, especially the more reddish purples, don’t appear in a rainbow. Aestheticists don’t join the two ends of the spectrum together–they add in other colors to complete the wheel.
