Spray paint would be fine. I'd recommend painting the outside, that way you have no risk of contaminating water once it's filled up, and if you decide you don't like it, it'll be really easy to just scrape off with a razor
I think he risk is not to the fish, but to getting the paint where you don't want it. spray paint goes everywhere and the dus will reduce visibiliy if you don' mask or seal off the rest of the tank. any latex paint will be easier to do and will provide the same color. I'd suggest a flat color for best adhesion. I mixed a little brown with black and got a nice velvety color on my new 20.
I used a roller on all of mine because they already had fish in them when I painted them. It worked fine. Actually even the one I did with no fish in it was done with a roller.