Afterimages are an optical illusion, which consists of an image persisting after one has ceased looking at a picture or light source. Positive afterimages are less documented, and are believed to be due to neural adaptation.
Conversely, negative afterimages are formed due to their imprint on the retina (the lining at the back of the eye). As outlined elsewhere, vision is achieved thank to photoreceptors, or rods and cones.
According to the opponent theory, cones, which are the ones that can register color, get less sensitive with prolonged stimulation and transmit a weaker signal.
As can be seen in the images below, red leaves a green afterimage, blue a yellow one and yellow a blue one. Black and white constitute likewise opposite colors. To see these, one has to stare at the colored square for 30 seconds – 1 minute, and then look at the white space. A corresponding afterimage should appear.
(this is a draft version for my lecture in Buenos Aires University, see more at Psychology in Images)
Afterimage Red Square
Afterimage Blue Square
Afterimage Yellow Square