I have all my tanks painted black, but I've been recently rethinking this. Something light colored to mimic blue sky in the background. It would depend what your doing with the tank and the scape but I think it adds alot of depth and a more natural look. When you look at a landscape photo (above water) the blue sky is an integral part. It's the same for an aquascape.
Look at pics of scaped tanks with light backgrounds, I think it makes a huge difference.