[Yodaime]

Originally Posted by shaq diesel

Rotrs will form a light rust covering if they get wet and are left alone. Once you drive and hit the brakes that is gone. If it's on your wheels then that's just that rust scale getting knocked off the rotors and landing on your wheels. A quick wash (or even a drive in the rain) should get rid of it since that rust scale is not nearly as adhesive as brake dust is on your wheels.

If it doesn't come off easy then it's probably some form of clay or dirt from something you have recently driven through.

+1