Why a ? Green Room?

Not so easy, this one.  Why do we always call the dressing room the Green Room when it isn't even green?  A typical reply: "No one knows for sure."  Theatre artists undoubtedly will continue to honor the tradition of the green room, despite their general ignorance of its history.  The most plausible reason seems to be documented in:

• 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 usage 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.

Some other reasons (guesses) put forward over time have been:

• It dates back to days when theatre was often outdoors, in Elizabethan days and earlier.  You went out onto the green to perform.  Or; the Green Room was just a canvassed area on the green grass adjacent to the portable stage at village fairs, or whatever.
• Originally such rooms were painted green to relieve the eyes from the glare of the stage.  This whole line of inquiry, however, is mooted by the fact that the term 'Green Room' is of seventeenth-century provenance.  Actors left the Green Rooms of that day to enter stages illuminated by candles, or oil lamps.  The intensity of stage lighting in the seventeenth century was considerably less than that of the twentieth, or even the soft glow of nineteenth-century gaslight, so the attendant effect on actors' eyes was correspondingly less injurious.  Although it cannot be denied that, as a colour, green is generally restful to the eyes, there surely is no connection between the choice of that colour and the glare of stage lights.
• The green in Green Room is derived partially from the American slang term 'greenbacks' (translation: money).  The Green Room is where the actors got their money for performing.

• There are two reasons for the Green Room being so called:

1.  Extras and minor actors didn't have their own dressing room, hence were green with jealousy at the stars for having their privacy; and

2.  Extras and minor characters were new (hence, green) to theatre and shared a common room.

What is clear from the early citations is that the usage of the term Green Room was not limited to a single theatre; but otherwise, its origins are obscure.

[Let me make one thing clear.  This summary is just a collection of realistic and not so realistic reasons given in answer to your question by others over the years,  I have simply consolidated them here -- it is not all my own work.]

Like a lot of terms in theatre, there is a certain mistique, and I'd encourage their perpetuation.

Cheers from Oz.