We used black spray paint and didn't put water in the tank for over a week. Our lfs said not to put water in for two weeks, but since I was curing live rock for over a month, I figured one week would be safe enough.
When I researched this I found out that the best paint to use is Latex. People have even painted the back of their tank with fish and corals in it. After doing the research I decided not to paint the back of my tank, instead I am going to use dbl sided tape and hang cloth. That way I can remove it if needed to see behind my overflows and in back of tank. As well as being able to change the color if I want. HTH