Metallic colors

I'm currently working on the #17 DeWalt Ford Taurus. Many of the parts are supposed to be painted aluminum. The chassis has aluminum on the interior but the remainder is black. I first sprayed the black and then went back and sprayed the aluminum over the black. At the same time I sprayed the many other parts that needed to be aluminum. The aluminum sprayed over the black looks far better, and was easier to see any spots that I had missed. My question is: when spraying aluminum, do you generally spray a black primer first or does spraying over black give the wrong color? I also need to paint some parts steel and was wondering if the same holds true for that color as well?

Thanks,
Chris