The hub is inside the rotor you aren't getting rust from that on the wheels that's why I figured you were talking about the rotor. Also as mentioned above almost certainly any discoloration on the wheels is from brake pad dust.

There is an new style of brake pads that are low dust. They don't brake quite as well and specifically don't have great modulation ability (where you can slightly relax the braking with Precision). If you don't push your car very hard you should look into the low dust ceramic mix pad.

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)