“Impossible colors,” or “forbidden colors,” are hues that cannot be seen by the human eye in ordinary viewing conditions. These are actually simpler to describe than one might think.
In our color wheel, we have colors like “red-orange” or “blue-green.” These colors exist because they are right next to each other on the color wheel. An “impossible color” is a color such as “green-purple,” or any two colors put together that are not next to each other on the color wheel. Though the idea of mixing green and purple makes most people think of a brown color, this is actually referring to a color that would be similar to both purple and green.
Other colors outside of our range of color perception are also included in this. Included is an image of two colors that are dissimilar. Some people who stare at the image with crossed eyes so that the pluses align may be able to see “yellow-blue.” Try it yourself!