There is a spray caliper paint available at places like Pep Boys, but what I did was buy a kit from Eastwood made for the purpose. It's a two part high heat epoxy paint. Once you mix it you have to use it quickly as it hardens quickly.
It comes with everything you need and in many colors. Check their web site. It's not at all difficult to use and is quite durable.
I used "Cast Grey" on my calipers and when my rotors were new I masked them off and sprayed all the surfaces not contacted by the pads with Silver High heat caliper paint. no more rust issues and the paint appears to be holding up to repeated braking