Originally Posted by
LarrySellerz
Are you suggesting that the discoloration is an oxide layer or something similar, and the reason it is only on the inside is because it’s being worn away by the braking? I think you are wrong, but that is an interesting idea that I hadn’t considered.
I think genejocky's correct; the discoloration is an artifact of heating, the friction of pads against the metal along the disc rim keeps it from remaining there. Inside the pad path the heat leaves the telltale discoloration.
Rotors changing color
I'd look for warp in the discs showing this coloration, may be why pads are rubbing. Heat could also come from pad friction if they're improperly adjusted, something you have some control over. If they're warped from
excessive heating you may want to source replacements before riding – and braking
hard – over too many more miles.