I working on a series of 12 skeleton paintings, and currently have 6 of the 12 in various stages of background development. When I first started I thought these would have just a dark plain background, but as I looked at them, they seemed to need more elements to make an interesting background.
Several of them will have curtains in the background to provide color and compositional effect. Here is an example. The reds on this one turned out really nice. I've been looking at lots of pictures of theater curtains, and a friend of mine has red velvet curtains. I took some reference photos to get a better understanding of the color, flow of the fabric, and shadows.
I'm going to add elements such as cages, staircase, tiled floor, etc. to make the other paintings work compositionally.