I did this today with red paint as my calipers where already red but faded and looked like junk.
i took the wheels off and then sanded it with corse sand paper i got from work hehe, the used really fine wet/dry paper i also got from work. and the after i done that i prepsoled the whole area to get rid of dust and rubbing down mess. then masked it all up and went ahead and sprayed them.
i must say they look hell better now.