You don't paint the whole rotor
Just the portion up to the wear line. You really want to do this on a new rotor and if you want it to look "new" all the time you can actually have the rotor zinc plated. I did this on my lexus and it looks great. Big 19" wheels that let you look into rusty rotors is not good, but get them zinc plated and they always look like they were just installed. IF you aren't buying new rotors then just use steel wool to get them shiny (not on the area that is going to come into contact with the pads though). Then paint them silver, anything else will be too ricey for a Porsche. Any caliper paint will work fine, just go slowly and don't expect it to look perfect.
Just my $.02
-Chris