Well, it depends on the color you're painting, and the type of paint
you're using. If you're talking primer, a lot of people use white. I've
used everything, from black, to white, to red, to gray, and I haven't
had too many problems, but I would stick to as neutral a color as
possible.
For actual undercoats, the color should be an approximation of your
final coat, if you're going to paint over it. If it's going to be your
base coat, it will need to be your basic color. Regardless of the
medium you chose to paint in, I would suggest using Vellejo acrylics as
your undercoat. If you chose to paint in acrylics, use Vellejo as your
main paint too, but if you're using oils or enamal, the acrylic
undercoat will lend its tone to your final coat, and won't soften and
blend with the base coat, like an enamal undercoat would.
Vellejo, and forgive me if you've heard the pitch from one of us
before, is specifically designed for figure painting and for use with a
brush. Great paint for figures. I am all thumbs, when it comes to
painting with them, but I use them form my undercoats, and I love how
they perform there.