The walls in my projection room are royal blue. When I had them painted, people kind of looked at me funny but, after we got the room done, people liked it.
When the lights go out for projection, they are indistinguishable from black and reflect nearly no light. When the lights are on, they don't look like a dark cavern like black paint would have.
Everybody else was pushing for black but we have a few other rooms that have black walls. It's actually difficult to work in an all-black room. There is no detail in the walls. That makes it difficult to gain perspective and determine where objects are in the room, relative to each other. Blue walls give enough detail that it's comfortable to work.
In my darkroom, the walls are wood paneling. Medium brown. I figure good enough.