I didn't say anything about painting the real rock, but you could. I was trying to suggest that you could paint the foam to help it look more natural than just the plain foam. Hopefully, though, it won't take too long for it to get some coralling algae.
By the way, it won't hurt any of your saltwater fish. My father is a marine biologist and some of their tanks have had it for more than ten years. My own saltwater tanks have had it in them for 2 years and contain tangs, angels, blennies, gobies, clowns, wrasses, firefish, and a sargent major.
Have fun and good luck