Any kind of paint will work. You want to paint the outside of the back panel. I used black spray paint. If the tank is inside I would brush or roll on the paint. I scratched the paint when I was messing with the hang on back filters so I touched it up with a piece of electical tape...
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+1 on the spraypaint. Go for some Krylon Fusion or something a little higher end, just for the pure reason that it's going to be a little more difficult to mess up (though still easy). I used $0.99 cans of generic blue on a tank once, and krylon fusion on another, and the krylon fusion is noticeably harder to nick or chip.