Use a dark gray for wheels---don't even worry about "correct color" or any pre-mixed concoction. Just mix up some black + white and go to town. If it comes out too light for you, hit your wheels with a black wash. The variation in tone only adds to the presentation. On another level, if you're going to weather your wheels with dirt, dust, etc,. don't even worry too much about the black color or shade. It's all going to be tinted by the weathering anyway.