I actually have a lot of experience with this, and didn't get it right until my fourth tank.......
Any spray paint will do, remember to mask the tank off and make it look professional.
DON'T GO WITH BLACK.... I know black is the common choice, but black is also a common color in many fish. To make the fish and corals POP, paint the back a <span style="color: #696969;">DARK GREY </span><span style="color: black;">. </span>
After 48 hours (cure time) use either a rubber glove or plastic wrap on your hand and evenly coat the painted surface with a <u>very thin layer</u> of tube clear silicone. You'll understand why after a year of salt creep eats away at the paint if you don't.
Even with the silicone covering the paint, you can scrape it off when you want by using a razor blade meant for scraping. I tried using a putty knife and made two noticable scratches on my tank, so be as professional taking it off as putting it on.
I only removed the grey paint because I needed to be able to view both sides of my tank, but this method lasted several years and looked good.
After