While I love the color black (and other dark colors) on a car, wow is it nice owning a silver car! I haven't washed mine for over a month and it looks exactly the same as it would if I washed it. Sorry, I didn't help you one bit but at this point in life I don't have the time to clean a car like I once did. Come retirement, I'm back to black!
Good luck in finding a good solution, I am envious of your car color.