There are several theories how the green room is called just that. Regardless of origin, the green room is where actors and TV guests relax before they go onstage. There may be food and drink provided and comfortable furniture.
An early explanation dates back to 1599 and the Blackfriars Theater, which offered a room behind the scenes where actors waited to hit the stage. That room was painted green and was even called "the green room," according to reports
In 1662, London's Cockpit-in-Court theatre had a room covered in green material, which may also have started the use of this term. Some suggested that these rooms were painted green so as to "relieve the eyes of the brightness and glare of the stage"
Others have suggested that the term derived from "limelight". This type of light refers to to calcium oxide which whose light was greenish. However limelight term was invented in 1820, which was at least 200 years after the 'green room' was coined.
In medieval theatre, the acting area was referred to as "the green." It was a central space, often outdoors and if so covered with grass. It was used by actors, surrounded by an audience seated on benches or simply standing surrounding this green area. "The green," then, became the stage the stage. When the stage 'moved' indoors it was still called the green. The green room would be adjacent and was where actors waited to for their cue to come on stage.
Also in medieval theatre, at green stage cloth was often used to inform audiences that a tragedy would be the subject of the day's performance. The green room could have been understood as the room meant for those players who perform on the green.
Green rooms may have been painted green on occasion in the past. but this is not the case in contemporaneous theatre.
https://www.theatrecrafts.com/pages/home/topics/stage-management/theatre-building/
https://americansongwriter.com/behind-the-history-and-meaning-of-the-term-the-green-room/
