I notice that when I use the MB wheel cleaner the brake disks show a rusk-type color (I have AMG wheels). This disappears after I drive the car (and use the brakes). In addition, after several days of rain, with my CLK under a carport, the brake disks show this same coloration.

Has anyone else had this happen and, if so, it is normal? Any recommendations?

It is normal and nothing to worry about.