As we've already discussed, the colors we perceive as red, green, yellow, burnt sienna and so forth are the result of reflected light being detected by cones in our eyes and then processed by our brains. To understand why so-called impossible colors break the rules of visual perception, we need to understand more about how our cones and our brains interact.
Each of your eyes contains roughly 6 million cones concentrated in the center of the retina [source: Pantone]. These cones come in three different wavelengths: short, medium and long. When a cone receives a strong signal in its wavelength zone, it sends electrical impulses to the brain. The brain's job is to combine the millions of electrical signals from each cone to recreate a composite "image" of the true color.
The brain, of course, is not a computer, but has its own complex lump of highly specialized cells. The cells responsible for processing the electrical signals from the cones are called opponent neurons [source: Wolchover]. There are two types of opponent neurons that reside in the brain's visual cortex: red-green opponent neurons and blue-yellow opponent neurons.
These brain cells are called opponent neurons because they function in a binary way: the red-green opponent neuron can either signal red or green, but not both. And the blue-yellow opponent neuron can signal either blue or yellow, but not both.
When you look at a pure yellow image, the yellow portion of the blue-yellow opponent neuron is excited and the blue portion is inhibited. Switch to a pure blue image and the blue portion of the opponent neuron is excited and the yellow is inhibited. Now imagine trying to see an image that is equally blue and yellow at the exact same time. The opponent neurons can't be both excited and inhibited simultaneously.
That, my friend, is why blue-yellow is an impossible color. The same is true for red-green. You might be saying, "Wait a second, I know exactly what yellow and blue look like together — it's green! And red and green make a kind of muddy brown, right?" Nice try, but that's the result of mixing together two colors, not a single pigment that's equally blue-yellow or equally red-green.