2018-12-21:
Impossible colors

Impossible colors are color perceptions which are not possible for the human eye under normal lighting conditions.

There are several kinds of impossible colors: Chimerical colors can be seen by looking on an image like the one on the right for at least 20 seconds, until some of the color cone cells become fatigued, and then looking onto another base color.

Mixtures of opponent hues, like green-red or yellow-blue can be seen by crossing eyes on images like this or this.