Outside only, inside needs to be bare.
Consider the heat. They get hot. No, hotter than that. Really hot. Nearly glowing red levels of heat. What paint are you thinking about using?
Few weeks ago there was a picture going around of someone who painted there brakes (disk). The painted the friction surface. The paint melted and solidified to the brake pads. Once cooled, it locked the pad to the rotor. Had to drag the car away. That is why you don't paint the friction surface.