I just did the back of my new tank in a dark blue krylon. Everyone says to use spray, but I brushed mine on. You do see brush-strokes when looking at it from the back (or the front) when you have a light source on the other side. Can't see the brush strokes really once you have the tank back against a wall. Besides, you'll have the back covered with algae before long. The nice thing about brush-on is that you don't overspray all over your living room, and you don't need to get somebody to help you haul the tank to a paint safe area if you have a larger tank. Alternatively, you could roller it on as well.
I'm partial to blue paint (darker shades), but others swear by black. Either should look nice, and like Civicman86 said, be prepared to do multiple coats. Give it a little time to dry between coats and you'll get a more uniform color.