The Troxler Effect

Stare at the black cross in the middle of the image.

Notice that the rotating vacant spot will begin to turn green and then the pink dots will begin to fade from view. The green happens because of an effect similar to that of the afterimage seen from a camera flash. When we look at a color for a period of time and then the color is suddenly taken away, our eyes tend to see an afterimage of that color perceived as the color's opposite. In this case, the opposite for pink would be pale green.


But the fading of the pink dots in our peripheral vision is an effect known as the Troxler Effect.
From Wikipedia:

Troxler's fading or Troxler's effect is a phenomenon of visual perception. When one fixates a particular point, after about 20 seconds or so, a stimulus away from the fixation point, in peripheral vision, will fade away and disappear. The effect is enhanced if the stimulus is small, is of low contrast or equiluminant, or is blurred. The effect is enhanced the further the stimulus is away from the fixation point.