Epoxy primer is a good general-purpose primer. It can be applied to bare metal and, unlike a surface primer, it resists moisture once it's cured. This is beneficial if you don't have time to spray the topcoat right away; you can leave your car outside in epoxy primer and it won't absorb water if it gets wet. Usually epoxy primers are available in light grey, neutral grey, and dark grey. Obviously, it's best to use a light primer if your topcoat is a light color.