What is colour constancy?

Colour constancy is the phenomenon that most colour surfaces appear to retain their approximate daylight appearance even when viewed under light sources that differ markedly from daylight. Colour constancy is surprising since the spectral distribution of light entering the eye from a surface can vary markedly from one light source to another. The phenomenon of colour constancy is only approximate, however, and surfaces do not retain their daylight colours when viewed under certain fluorescent light sources or when viewed under monochromatic radiation. Certain surfaces appear to change markedly from one light source to another and such surfaces are said to lack colour constancy; this phenomenon must not be confused with metamerism which is a phenomenon associated with at least two samples.