Early computer displays were monochrome and used what is known as a cathode ray tube, or CRT. Instead of red, green, and blue pixels of current displays, CRTs scanned an electron beam (the cathode ray) across a phosphorescent screen. When electrons strike phosphors, it causes them to fluoresce, thereby producing a visible image on the screen. The image can be changed by adjusting the scanning pattern of the electron beam.

Phosphors can be made in many colors, not just green. But green was chosen because it had a few advantages over other colors at the time. One of the most important was the persistence time, or the time that the phosphors remained illuminated. This is because humans perceive motion from still images if those images are incrementally changed over short periods of time, which means the screens had to be updated several times per second. But if the persistence time was too low, the phosphors would go dim before being refreshed and the screen would appear to flicker.

The green color happened to have the longest persistence time and therefore did not appear to flicker. And although green text is what most people remember, amber phosphors also had a long persistence time and were used for these early monitors.