I've always used Rustolium Flat Black. I'd like to try a dark grey though, that's an interesting idea.
If you tape paper on, use packaging tape and do a full line across to protect against salt creep.
I once tried to roll on white paint for a client with a 220g African Cichlid tank. It ended up not coming out nearly as well as spraying it on. And I know, everything shows up on white, but that's what he wanted. White stand, white back, white substrate, and white rocks. After installing a large U.V. there really wasn't a problem anymore.
