FYI

Legend has it that the green room, also styled greenroom or green-room, goes back to the Elizabethan era, when the actors lolled away the time between entrances on the lawn behind the theater, or "on the green." Alas, like so much else in show biz, this appears to be a crock. According to my Oxford English Dictionary, the earliest known usage of green room was in 1701. One plausible theory is that the green room was originally painted green to rest the aching peepers of the actors, who were bleary-eyed from the bright stage lights.

- Taken from The Straight Dope