I'm guessing you will have to repaint them fairly regularly just because the surface or the dead corals is soft and will gradually dissolve. Just like painting cement. If it has water running over it, it lasts pretty well, but immersed you have to repaint fairly regularly. Like the basin in a cement fountain.
Back in the day the look that everyone liked was the pure white of decorative coral skeletons (see tanks in movies from the '70s and '80s). So we bought large skeletons to decorate and pulled them out and bleached them regularly. It does give a sharp clean look if rather unnatural. If you wanted color you could get red Tubipora Musica skeletons or blue skeletons from Pavona (I think)
The bright white decor really contrasts the fishes' colours and makes them stand out. IMO painting old skeletons will just draw the eye away from the fish that should be the focus of your system.
