Color vision and the four-color-map problem

Four different colors are needed to make maps that avoid adjacent countries of the same color. Because the retinal image is two dimensional, like a ma p, four dimensions of chromatic experience would also be needed to optimall y distinguish regions returning spectrally different light to the eye. We t herefore suggest that the organization of human color vision according to f our-color classes (reds, greens, blues, and yellows) has arisen as a soluti on to this logical requirement in topology.