I keep my Crossfire wheels clean so I can't comment on them personally. I would try some brake dust cleaner and let it soak for a while, repeat as needed but be careful when scrubbing.

I helped a friend clean her wheels on the new style Bug. She NEVER washes them and then decided one day when she was over and asked for help. We literally scrubbed the paint off the wheels in the bad spots. I don't recommend that but she didn't care. In a week you couldn't tell again anyway