Why paint? That's very permanent. I have a reverse sided backing (from the LFS) that I just used tape to attach to the back of mine; one side is blue and the other is black. When I feel like a change I remove the few pieces of tape and switch. Currently it's on blue, but only because I had to seriously scrap the back of my tank for the black to work. And yup, I like the back of my tank nice and clean.