For armor, I use flat black. When I put an uneven topcoat on, I already have a working faded look. As a general rule, however, the determinant factor in primer color will depend on the topcoat you want. For very light or potentially translucent colors, use a light primer; in many cases white is the color of choice. For darker colors, a gray primer is would be better (gray automotive primer, for example).
Gip Winecoff