I first came across the notion of “forbidden” or “impossible” colors in a horoscope. The idea of there being mythical colors beyond our range of perception is an intriguing notion, but I assumed that, like astrology in general, it was a little scientifically flimsy. However, it turns out that at least a few scientists have been studying the outer limits of visual sensitivity, and designing (somewhat controversial) experiments to temporarily induce the perception of these elusive “red-greens,” “blue-yellows,” and “supergreens.”
Basically, these experiments attempt to exploit the fact that we see color with photoreceptors that individually can register either red or green, and either blue or yellow, but not both red and green or both yellow and blue simultaneously. One experiment flooded study participants’ vision with red and green (or blue and yellow) stripes using a special retinal tracking device that ensured that individual cells were receiving only red or green input, thus overloading the photoreceptors in a way that reportedly produced an unnamable color that was at once red and green (not to be confused with brown). Another experiment can be done at home by just staring at a red/pink field on a computer screen and then shifting your vision to a blue field and taking in the “supergreen” that results from super-fatiguing the red cone photoreceptors.
I can’t think of a lot of applications for this phenomenon beyond stirring the hearts of romantics and extra-perceptual thrill seekers, but apparently someone in the Walt Disney empire could. According to this breathless post about impossible colors, the pavements at Disney World are a particular shade of pink that tires out red receptors to the point of making the surrounding landscaping seem extremely green. Huh.