Place your mouse pointer next to the fixation cross, and steadily fixate on the cross for about thirty seconds or more. Try not to avert your eyes. Then, click on your left mouse button, and the colors will disappear. You will see ghostly images of the complementary colors.

So What's Going On?

    Color afterimages are  similar to black and white afterimages. They are caused by fatigued cells in the retina responding to light. The most interesting color afterimages are  negative afterimages. If you stare at the red color for 30 seconds or more, the cells in your retina that respond to red will fatigue and will fire less. When you  switch over to a white surface, your eyes subtract the red and you see its complementary color green. 
 

     Color is first encoded at the level of the photoreceptors in each eye.  There are three types of color sensitive photoreceptors ("cones"). Their job is to convert the incoming light into electrical signals that the rest of the brain can use. 

     We have three types of cones, maximally sensitive to  red (R), green (G)  and blue (B). A red stimulus will tend to activate mainly, but not exclusively, the red photoreceptors, green light the green cones and so on. 

     The output of these receptors is converted in the retina into an opponency process.  In this process the output of the eye, the one million fibers making up the optic nerve, encodes color in three separate channels, one for intensity and two for color. One set of neurons encodes black-white differences, corresponding to intensity or luminance  differences in the image (similar to looking at a scene through a black-and-white videocamera). Another set  of neurons responds to red and green color differences and a third set responds to yellow and blue differences. For instance, a red-green cell would increase its activity as a result of stimulation with red (R) light and would decrease it's activity in response to green (G) light. It can be said to signal +R-G.  Other cells signal the opposite, that is the presence of green and the absence of red (+G-R).  A blue-yellow cell would signal +B-Y (some signal +Y-B),  while luminance cells  signals something like the weighted sum of R, G and B.  Our subjective feeling of color depends on the relative activity in these three sets of neuronal fibers.
 

     Interestingly, the NTSC standard for  television transmission in the United States uses a very similar system with one luminance and two chrominance channels. 
 

     Once we know about this opponency processing stage, first proposed by the 19th century German psychologist Ewald Hering on the basis of perceptual experiments, the explanation for the color afterimages seen here is relatively simple.  As in other types of negative afterimages, when you stare at a red stimulus, the cells signaling the presence of red will start to fatigue. Thus, when looking at the empty screen these cells will now fire very little. However, because they normally encode through their activity the presence of red or the absence of green, reduction in their activity is interpreted by the brain as the presence of green.
 

     Thus, you see a green afterimage. The same applies to the other colors you see: the green will be replaced by a  red afterimage, the yellow by a blue and the blue by a yellow afterimage.  As you continue to observe the afterimage carefully, it fades and its color changes slightly. This is because your different cones (and chromatic mechanisms) recover from adaptation at different rates.
 

     Complementary color afterimages are explained in terms of the neuronal processing in the retina; the fact that neurons encode color in terms of opponency processes.  In this manner many of the interesting visual phenomena and illusions associated with the viewing of colors are accounted for.