Blue shadows: physical, physiological, and psychological causes.

The apparent blueness of outdoor shadows has two main causes: the illumination of the shadows by blue skylight and the enhancement of the perception of blue by simultaneous color contrast. Other physiological mechanisms, such as brightness contrast and afterimages, can also affect the perception of a shadow's blueness. Preferential scattering by the cornea does not seem to make a major contribution. Despite these effects, color constancy causes most people to observe an empirically blue shadow as colorless.