Primer can be just about any color, and of course, should be flat. The most common primer colors that I have used or heard about are red (rust color), varying shades of medium grays, and white. If your top color is going to be light, then a light primer is suggested. The opposite may be used for darker colors. The bottles don't actually say "primer" on them, it's just that the first color next to the plastic is considered the prime coat. Prime coats can be acrylic,enamel, or even lacquer (Lacquer has to be sprayed on in several mist coats to keep from etching the platic too severley. You need tooth for the base coat, but you don't need a surface that looks like 20-grit sandpaper.)
I spray my models flat black, then begin to dot-in the base coat with my airbrush instead of spraying on a complete coverage coat. Dottin-in is kinda like spraying small, overlapping polka dots, but doing it so that the black subtly shows through when finished. It takes me an hour or two to put the top coat on a 1/35 AFV. Once completed, I'll mix a little flat white into the base coat, and start dotting-in again, but this time I'll only spray toward the inner portions of panels, hatches, etc., leaving the outer panel lines, and weld surfaces darker. This gives the appearance of faded, sun-bleached surfaces. But the black still show through very subtly. Then on to the wash and dry brush, but that's another time and topic.
Hope this helps you some.
Gip Winecoff