My suggestion is, if you do use the light sealer, is turn your lights off before you clear, and use a handheld light to run over the whole car looking for light spots. Concentrate on rocker panels, bottom of fenders and quarters, and other nooks and crannies that are hard to spray. There is nothing more disheartening than to clear your job, then pull it outside and see light spots everywhere. Unless you have really good lighting, I would suggest at least 50% black and 50% gray. It is true that spraying a dark color over light sealer will let you see how even you are applying the base, but it will also bite you if you happen to spray light in the same area every coat. It will look covered until you pull it outside, then it's a complete redo. To get COMPLETE coverage, a LOT of colors require 5-6 coats or more, evenly applied. You may try spraying your color on a sprayout card before spraying the vehicle, just to get an idea of the coverage of your particular color. When you think it's covered, hold a light behind it to verify it. Chances are you will be able to still see the difference between the dark and light areas. To me, it's not worth the risk spraying over a much lighter sealer. Sealer is cheap compared to color, and is a better product. Too much base can cause a multitude of problems, besides the cost factor.