On my current project I sprayed the entire boat with gray primer. The bulwark is going to be white. Well, the white satin I was spraying on the bulwark was not covering the gray primer properly. I sanded off the paint down to the fiberglass, sprayed with white primer, sprayed the white satin and all was fine. From now on with the paint these days light color spray paint gets white primer and dark colors get gray primer.
As a test I sprayed scrap materials, one with white primer and one with gray primer. Then I sprayed Navy Blue over each. The Navy Blue looked nice and dark and even on the gray primer. The blue on the white primer was light in color and it did not cover the white very well.
My 2 cents on this.