Why are stage curtins red?

Why are stage curtins red?
The colour red of the curtain and the gold trim that many used were seen as "expensive". This gave the impression of success. The seating in theatres were mainly of a red colour to match the main curtains. These were highly visible and looked expensive when the house lights were up and did not reflect any stage light when the house lights were dropped. Red was also a reasonably inexpensive colour to manufacture as a colourant. Added to this was the effect of the lighting on the materials. Light colours marked very easily and these marks showed up under the front of house footlights. The red was a medium colour that hid much of the marks.

Now many theatres have differing colours of which a large percentage are made to enhance the theme of the show on at the time. Some use high powered front projection to put a relevant logo of the show being seen.