Eastwood sells a brake caliper coating system for $36. It's a urethane coating that should work better than spraypaint, but I've never seen it in use so I can't vouch for it. They have several colors available.

http://www.eastwoodco.com/shopping/p...emType=PRODUCT

You might also want to check with someone like HPC about getting the calipers powder coated. You'd have to take the calipers off and send them in, but there is probably someone in your local area that can do powder coating as well.

http://www.hpcoatings.com/appearance_coatings.htm