Discoloration from heat is anywhere from straw to black. The stages are: light straw-straw-purple-blue-black. Brown is hydrated iron oxide. I think you're looking at rust.
Does the car sit idle for long periods? Brakes suffer first on a car that sits unused. If the shiny metal doesn't come back on the rotor surfaces after running it for a couple weeks, then check into the possibility you may have a seized caliper.
95 E320 Cabriolet, 131K