I have a paper background on the back that I got from my LFS. It was pricey, like $2 a foot, but it looks great. It's dark blue at the bottom, and gets lighter as you go higher. I used vaseline to stick it on the back, and it works great. Vaseline takes FOREVER to dry out, so I'm not worried about it falling off.
I would not paint the back if it's set up already. Too great a risk for airborne toxins to find there way into your tank/sump.