First, if you wind up experimenting, avoid water-based paints.
Most epoxy paints, once dried for *several* days, preferably in a hot sun, should be plenty safe. You can also find (sometimes) black PVC pipes that are usually less obtrusive. You want something that dries nice and hard, almost like a shell. Be sure that the indications on it says it bonds to plastic. SAND the PVC pipe with something like 400 grit sandpaper to make it a little rough before you paint it (which usually makes it look more natural anyway). Avoid anything that requires a primer.
Try Rustoleum, but let it dry for several days until you can't easily scratch it with a fingernail.