Often you hear Hosts say that their celebrity guests are “in the Green Room” before being interviewed. (The Green Room hasa drinks and snacks for the celebrities) I noticed that Ricki Lake often puts guests in “the Green room” so they can’t hear what the other guests are saying.
How did the “Green Room” come to be? Was it/ Is it actually Green?
It’s a very old theater tradition. The green room was where actors would wait when their presence wasn’t required on stage.
They were usually painted green, but it’s unclear whether they were named because of the color, or colored because of the name. One suggestion was that greens were scattered around the room, possible to muffle the sound of footsteps. There doesn’t seem to be a definitive origin.
I’ve seen one source claiming that the Green Room is where the actors would “turn green” from stress or jitters before going on stage (unlikely, as this is where actors change clothes - waiting is done in the wings). I suspect that some old and famous English theater had a tiring room painted green, and it just spread.
My drama teacher said it was bad luck to color the Green Room green. Of course, he thought everything was bad luck. Bloody thespians. 
My high school drama teacher told us it was because “that’s where the actors get paid.”
I said: “Was the money really that color back then? Didn’t they mostly use coins?”
I didn’t get an answer.
I suspect the actual derivation is lost in the mists of time, and all we have are the best-guess theories as quoted above.