"Krylon Fusion for Plastic" spray paint. (i bought mine at Walmart)
I used this on my PVC pipes in my overflow and on my return nozzle thing. I've had it in my tank for about 7 months with absolutely no problems.
Best way to do it in my opinion is get some sand paper and very lightly sand down the surface so the paint sticks better (you might not even have to do this b/c the product says you don't need to sand, but I like to anyways). Let it dry for a day or 2 (says it dries in 15 mins, but I'd wait longer just to play it safe). Rinse it well in warm water and you're good to go.