If the tank is not setup yet, any paint will do. I prefer a medium to dark blue. Spray paint will give the best even coat. A clear coat of polyeurathane (sp?) or laquer will protect the paint from scratches from magnetic algea scrappers and the like.
If the tank is setup use acrylic paint. This is water based and therefore has no harmful fumes. Do not buy acrylic paint as spray paint, it has harmful propelants.
Some people will tell you they like a black background. I would say it's up to what color fish you have. Any dark blue or black fish will blend in with the background too much for my taste. Also realize the darker the background the higher the possibility of it absorbing heat energy from your lighting and releasing it back into the water. HTH