What Are The Colors You See When You Shut Your Eyes?

Sometimes, when you shut your eyes tightly, your eyes decide it's time for a light show. Random, psychedelic-like patterns and flashes of color will dance around your field of vision. While you can appreciate the entertainment, in the back of your mind you wonder what is causing the phenomena.

Are these flashes and patterns of light and colors indicative of a health problem? Are they something that can be treated or cured? Or are they completely normal?