When you look at the two skulls in the image, you would seem to see a purple and orange skull but actually, they are both red. Only the striped background changes our perception of their color.
The pigments morph because of the Munker-White illusion, which shifts the perception of two identical color tones when they’re placed against different surrounding hues. No one knows for sure, but the illusion probably results from what David Novick, a computer scientist at the University of Texas at El Paso, calls the color-completion effect.
The phenomenon causes an image to skew toward the color of the objects that surround it. In a black-and-white image, a gray element would appear lighter when it’s striped with white, and darker when banded with black.
(Image credit: Popular Science)