You probably want to use a high temp black spray paint, often marketed as BBQ paint. You can pick this up at almost any hardware store. The best way to paint your fixtures is to remove as much of the case as you can and spray the individual pieces. If you can't remove parts that need to be painted, make sure that you cover and openings/joints/moving parts,
etc. and then spray them. You don't ned to use a lot of paint, only enough to make a nice coat.