I always thought it was like a 'waiting room' to the stage, and thus more for actors. (don't TV talk shows use Green Rooms for guests?)
About.com had this to say:
From Maria Knapp,
Your Guide to Theater.
Definition: The Green Room is the social room for the actors to hang out and relax in. If an actor has a long time between times on-stage, she may choose to wait in the Green Room rather than the dressing room, which can be crowded. The Green Room was named this because when theatrical lighting was in its early stages, lime was burned to create a very bright spotlight. (Thus the term "Limelight.") It blinded the actors causing them to see green spots in front of their eyes. They were lead to the Green Room until their vision cleared and they could see again.
And this from World Wide Words:
[Q] From Bob Lallamant: “I have always wondered why the reception room for performers—opera singers, actors and the like—is called the green room?”
[A] Originally, the term referred to an off-stage room in a theatre where actors could rest while they were waiting for their cues. A lot of theatres now don’t have green rooms, often through lack of space. These days, the term applies as much—if not more often—to the reception room in a television studio where guests wait before appearing.
Why it should be called a green room is a minor mystery. The first recorded use is in a play by Thomas Shadwell called The True Widow, first performed at Dorset Garden Theatre in London in December 1678: “No, Madam: Selfish, this Evening, in a green Room, behind the Scenes, was before-hand with me”. The use of a here might suggest it was just a green-painted room, but a slightly later example, in a book called Love Makes Man, written by the actor and dramatist Colley Cibber and published in 1701 makes the usage clear: “I do know London pretty well, and the Side-box, Sir, and behind the Scenes; ay, and the Green-Room, and all the Girls and Women Actresses there”. Colley Cibber was closely associated with a different theatre, the Drury Lane.
It has been suggested that the room was painted green to rest the actors’ eyes after exposure to bright stage lighting, but in the early 1700s, when lighting was by candles, that could hardly have been much of a problem.
In an article in De Proverbio, an online journal of proverb studies, George B Bryan points out that the colour green has long been associated with the theatre, perhaps originating in the liveries worn by members of acting companies in the time of Shakespeare. Mr Bryan also records that green baize was sometimes used to cover the stage at this period to protect the costumes of the actors.
This is a possible origin of an associated usage, the green, for the stage itself, which is still sometimes to be heard. The direct connection seems to be twentieth century rhyming slang: greengage = stage, but there may be a subconscious—or even a direct—link back to the colour of the stage covering.
What is clear from the early citations is that the usage was not limited to a single theatre (and so was not an accident of paint colour or wall covering) , but otherwise, its origins are obscure.
I think for a TV show or large/professional company it is for actors, but for small theaters it should be for anyone, if you have one. Just my opinion.
Our theater doesn't have one.