Paint it with silver high-temp paint or just live with it.
Whoa! I would never recommend painting over rust untill you get rid of it first. Try taking one of those wire brush things we all use on grills to see if you can scratch most of it off. Or you could try steel wool, just to get rid of it. Be careful, if you decide to paint it, try to make sure that no paint gets on the contact surface of the disc itself. If none of that works, then you could dish out a few pennies to pick up a set of aftermarket drilled and slotted rotors!