Jay my experance with paint has been the under coat color. Is you want it to be darker use a dark primer such as black or red. If you want lighter go with gray or white. I use Dupont systems and follow there recomendations. I used Omni on one of my GTOs and was not happy with the coverage, it was like spraying water and it wound up tranparent. After a resand and reshoot it came out better. This was a base/clear coat system. Omni is a Ditzler low end product, it was cheaper and not as good as the upper end product.  Also the sealer has the same effect on topcoats. My 56 F1oo is base/clear from dupont, Ihad one panel that is lifhter under shop lights, but in the out side light you can`t see the difference.