and regardless, with or without a gray underneath. you will end up with the same amount of coats of paint. say you do 6 coats of red, til it covers, but may be blotchy at some areas.
but say you do two coats of gray, and then say it takes 3 coats of red, the red will be uniformed, but when you count how many coats you put on, it's usually close to each other. but the gray then red will make it uniformed, and no blotches. reds and yellows and whites all all blotchy paint. the pigment seems to not spread that well, and when you to spread it, it becomes clear. kind of like cum, its thick in the middle, but surrounded by clear. you got the point?
but it is the best way with those colors to use the right underlying color first to have a uniformed paint job.