Pinda...
Painting the sides/back isn't going to reduce the amount of light you need. But if you're going FOWLR, you don't need much in the way of lighting. More lighting will make the fish color more intense, but they don't "need" the light like corals do. If you like the look of the 120W bulb, it should do fine. Melosu58 covered your lighting needs for corals.
Tank2379...
Built in reflector or not, even with a single 110W bulb, you're going to have a rough time keeping anything but some low light soft corals like mushrooms and certain zoanthids. Just FYI...