Problem with using regular spray paint (high temp, rustoleum, etc) is the paint is too porus and will eventually absorb all pad dust, dirt, oils, fluids, etc that the environment has to offer and after a while, they will not clean up nicely. You need to use epoxy or equivalent to get the non-porus coating that will shine and last forever.
Problem with powder coating is the application process is at around 350 deg F and you have to strip all parts and seals out of the caliper, the caliper then has to be chemically cleaned to remove any flammable brake fluids, etc. By the time this is all done, you could have purchased a brand new set of 4 each powder coated calipers for less money (~$400).
Solution - spend $75 and purchase 2 kits of epoxy paint from Mid-America. Yes, you are limited in colors but at least you can color the calipers and save the 'ol wallet. Mine are Torch Red to match my Torch Red car and the epoxy is holding up great.
SAVE THE WAVE