The Abney Effect is a perceptual change in hue that occurs when white light is added to monochromatic (one color) light. Adding light to a light of a single color is essentially desaturating the light and the altered hue we perceive is called the Abney Effect.
This effect is credited to Sir William de Wiveleslie Abney (1843-1920) who was a chemist, physicist, and photographer. When white light suddenly illuminates a surface it first appears to brighten in the middle of the surface followed by the edges of the surface. When the light is extinguished the edges of the surface darken first followed by the center.