Everyone has their own ideas about coverage and sealer colors. Some colors are very difficult to achieve hiding, new yellows that are chromium-free, for example. I think if someone is going to try and use a sealer to "help" coverage, first they need to do a sprayout on a black/white test panel with only the paint to see how many coats it takes to cover. If it's four or less it really doesn't matter what color sealer you use, the base will easily hide it. But if it's five or more, sometimes it pays to find either the right shade of sealer OR a high coverage "ground coat" made from a solid basecoat color that will help hide the sealer underneath. In these case where the ILLUSION of full coverage is to be attempted, more sprayouts need to be made with the sealer or groundcoat of choice, followed by the desired number of color coats. All panels should be cleared and compared to the first "pure" sprayout sample that was done with only basecoat. If the illusion of coverage has been achieved, the panels will look the same in bright sunlight, pay special attention to the "flop," the way the color looks at an angle. That is where our eyes can best see down through the layers of pigment, and any problem will show up.