Time and patience. I would not paint the rock. You may find that the paint will not be toxic to the fish but I doubt it would hold up long under SW conditions. Then you will have paint peeling and flaking off in to the water and the fish/corals may eat it and that would be bad. In time if your water conditions are good the rock will get color. You can also get yourself a small piece of coralline covered rock and use that to seed your tank and eventually it will spread to the other rock.