Color is the easiest channel to use to notice afterimages. There are two kinds of afterimages: negative afterimages, and positive afterimages. Negative afterimages have been discussed in a previous section where you can make a plain white screen appear colored.
Positive afterimages occur because of the property of persistence. Because chemical reactions are not instantaneous, the visual stimulation remains in the brain for a certain amount of time. Normally this amount is a small fraction of a second, so that when something moves we don't even notice the positive afterimage. However it is possible to force a signal to travel to the brain for a longer period of time.
For the same reasons that visual stimulations aren't instantaneously processed in the brain, they also do not instantaneously reach the brain. This concept, called latency, is a source of two other very interesting illusions.
The length of time between light first striking the eye and the brain responding to the light varies greatly upon the amount of light and the color of the light. The more light there is to process, the faster the brain will respond to it. Each of the three color channels has different chemical reactions so has different latency times.
The brain has compensated slightly for latency, which ironically produces another very interesting illusion. As the diagram above shows, the brain can react to an alternating pattern of on-and-off stimulations and, believing that they are caused by latency and the high frequency of light, translate them into a wave pattern. This wave is then processed to be a wave of light, and you see the black and white as color!
This idea was first noticed by a toymaker in the nineteenth century who created tops that took advantage of this trick. Benham's disks, named after the toymaker, are still popular at toy and novelty shops because of their fascinating properties.