Forbidden Colors!

Not actually "forbidden" in any real sense of the word, the color mixtures of red-green and blue-yellow are not easily visible to the human eye. That's because the wavelengths of these pairs cancel each other out when they reach the eye at the same time.

It's especially true with the red-green border -- blue and yellow can mix to show green and red and blue can mix to show purple, but red and green stay distinct in most cases. Apparently, the neurons which carry the "This is red" signal to the brain are shut down by light in the green wavelength of the spectrum. The same thing happens between neurons which register light in the blue and yellow wavelengths: The neurons that carry one signal cancel out the neurons that carry the other one.

A 1983 experiment showed a way that the forbidden colors can be perceived, if stripes of the two opposing colors were put next to each other and subjects stared at them for awhile. A special "eye tracker" held the surface on which the color was found stable relative to the eye, meaning that it was moved if the eyes themselves moved. The subjects' eyes thus didn't shift back and forth between red and green but instead could stay focused on the border. After some time, the border seemed to disappear and the viewers saw a reddish green and a bluish yellow -- the colors previously "forbidden" by the eye's structure.

Later experiments cast some shadows on the earlier ones, but the scientists working from the earlier set of experiments point out that their version used the eye tracker but the later ones didn't. One of them thinks that when the image is stabilized in front of the retina the opponent neurons no longer cancel each other out and the brain can process the dual signals in a way it ordinarily can't.

Which of course won't help you if you claim that your eyes caught the traffic signal as it changed and you couldn't distinguish between red and green, or even if the light was yellow, sir. Especially if the officers got SCMODS.