I was told that black would make the colors pop out more.
I am starting a 75G FOWLR myself, and I bought black paint yesterday. But, but, I couldn't, for the life of me, muster up enough courage to paint the back of my tank. So today, I went into Petland and bought me one of those backgrounds that they have (the one with blue on one side and black on the other). That should cure my indecisiveness.
Maybe you can try that.