Westleys is good, but some tires will turn yellow and they will always be yellow.
I have read that the white wall on some tires are placed on black tires and valcanized on to the sidewall. The black bleeds though the white rubber and discolors it forever.
Some tire companies do not have this problem, I have a set of tires from Coker that have not changed color and set of Firestones that didn't do it either. For black marks or just dirty tires Westleys is as good as anything, a brass brush will help it they are really bad.....