If we find two colors on a spectrum line, we can average their wavelengths to find what color they’ll produce when mixed:
Wait, the reality of color is a bit more complicated:
There are “real” colors (we call them pure spectral or monochromatic colors) and “unreal” colors that only exist in the brain.So what are the rules for creating these “unreal” colors from the very real photons that hit your eye? Well, in the 1920s W. David Wright and John Guild both conducted experiments designed to map how the brain mixed monochomatic light into the millions of colors we experience everyday. They set up a split screen — on one side, they projected a “test” color. On the other side, the subject could mix together three primary colors produced by projectors to match the test color. After a lot of test subjects and a lot of test colors, eventually the CIE 1931 color space was produced.
On the curved border we can see numbers, which correspond to the wavelengths in the spectrum we saw earlier. We can imagine the spectrum bent around the outside of this map — representing “real” colors. The inside represents all the colors our brain produces by mixing — the “imaginary” colors.
So, finally, blue and red make magenta, not green:
But it’s all in your head.