electron gun A color monitor is based on a cathode ray tube, or CRT. This tube is made up of three electron guns that produce a stream of electrons, each of which causes a red, green, or blue phosphor to light up. Controlled by electromagnetism, the electron guns sweep across the screen, their strength varying so that a colored image is drawn. Different colors are created by the additive color mixing of the red, green, and blue phosphors. See also: color balance, color purity, CRT