Question: In broadcasting, why is the guest room called the green room? Jim A., Jacksonville, Fla.
Televisionary: Boy, I'll bet you figured there was a simple answer for this one, eh, Jim? There ain't. In fact, the one definite thing I can tell you is that nobody really agrees on the origin of the word.
It predates the tube by quite a bit, though. While it currently describes the room guests wait in before appearing on TV, it was originally used in the theater world to describe where actors killed time before coming on stage. The first written use of it to describe such an area dates back to the early 18th century.
Now, why it's called that remains a disputed topic. Some people claim the room was painted green to soothe performers' eyes after they squinted into the bright stage lights (doubtful, since electric lights weren't in use so the stage lighting wasn't all that bright). Others claim that because the slang term for stage was "the green," it developed from that. And that's just scratching the surface of the theories.
In short, I don't know for sure. And I don't feel too bad about admitting that since no one else seems to, either.