I prefer Tamiya enamel primer, but many people use Krylon as well. Krylon is cheaper and available at K-Mart and stores like that. Tamiya seems to me to be a bit thinner and doesn't cover the surface detail as much. I have also read that a lot of people decant the Krylon primer into a bottle and spray it with their airbrush so they have better control,
As to color, I prefer the lighter colors such as white or light gray. Something with just enough contrast between the plastic and the first camo color so I can tell where I've painted. It doesn't really matter since the camo paint is going to cover it, but If you use something real dark, such as black, it may be a little difficult to completely hide with a light camo color..