LED

A light-emitting diode (LED is a semiconductor diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction. This effect is a form of electroluminescence.

LEDs are available in various colors most notably:
 * RED
 * GREEN
 * YELLOW
 * BLUE
 * WHITE

See; http://en.wikipedia.org/wiki/Light-emitting_diode