Simple Green eats paint. Don't use it. It's also a degreaser, which you don't want to use on any painted surface unless you're planning on waxing to re-add some protection.

Get a brand name, painted-wheel-safe wheel cleaner, apply to wet wheels, and don't let it sit on the wheels too long. Use a soft brush or rag to get the brake dust off, hose down the wheels really well when you're done, and wax them after they're dry. The wax will help protect the paint and also help keep new brake dust from sticking, the same way wax protects the rest of the paintwork on your car.