I just did this very project using Windex Green to clean he back. I taped off everything well, turned the tank upside-down so that excess paint wouldn't get inside and went to town. Several light coats of spray paint did the job. I used satin/semi-gloss finish rather than flat but that is personal choice. I read a bunch of articles and decided on dark hunter green rather than black. I've always used some kind of black background before, but my black angelfish disappeared! Dark green allows me to see my angels and gives tremendous depth to the tank. I have read article showing how different colors create different effects...tan/brown simulates a rocky wall, black simulates deep water, dark blue for ocean effects, green for planted tanks to provide the illusion that the tank goes on for long distances, etc. Black can look really awesome depending on your fish. It makes red, yellows and blues really pop.