Talino,
I paint in oils and this is just a personal preference but I generally undercoat in a lighter shade and for the very darkest points (behind the nostrils, under the eyebrows) I usually "block in" a darker color during the undercoat. The reason I do this is because I found it easier to make an area darker with oils, but more difficult to get a good highlight when using a darker basecoat. The blocking of dark areas backs up the darkest shadows so you don't have to use as much paint.
Lou