Answer: Phosphenes

Under normal conditions, the only visual stimulus you receive is from a luminescent source (however bright or dim it may be). When you rub your eyelids or otherwise exert pressure on your eyeballs, however, there is an immediate burst of brightly colored lights and patterns. This visual stimulus does not originate from an external lighted source but as a side effect of misfiring retina nerves; the effects are known as phosphenes.

Although pressure on the eyes produces phosphenes, there are other mechanisms that will also produce the same effect, including exposure to electrical current and strong magnetic fields, as well as fatigue of the optic nerves–if you stare long enough at a monocolor wall, you’ll begin to see bursts of color and other phosphene-related visual artifacts.