Brightness Constancy

Brightness constancy is when we perceive colors to be darker or lighter than they actually are based on the relative luminance. Relative luminance is the amount of light an object reflects relative to its surroundings. For example in the two pictures below the girls have the same skin tone, but the one on the left appears to have a lighter skin tone than the one on the right. Since the girl on the left is surrounded by grey it appears as if her skin color is lighter than the girl surrounded by white.