Unless the veneer is peeling back and the bare metal has begun to corrode, warm, soapy car-shampoo in solution with water will remove brake-dust (black) / disc-rust (reddish-brown) from alloys. Use one of those little nylon brushes to remove the stubborn bits and use an old sponge as well (never use this sponge on car-body afterwards) Power-hose wheels before hand-wash and power-hose afterwards.
Do not use acid-based cleaners, corrosive scrubs (pot-cleaners), CillitBang or other similar household solvents.
Once cleaned, polish & shine wheels carefully with car-polish. Be careful of pads and discs.