I think the contrasts are good, I know when I do darker skin tones it is something I find hard - I sort of get shadows and highlights and not much left in between to make a face. That looks good though and you were able to build up some nice skin tones!
I like the hatching here! Gives the skin a nice texture.
I think you might have concentrated a bit too much on trying to show all the subtle curves on the shape of the face - they seem a little exaggerated. If you're going more for realism, try thinking of it like this: the majority of the shape of the face is given by the skull. That means straight, solid lines when you#re constructing, except on fatty/muscle areas like the cheeks. Playing straight lines against curves gives a strong effect for describing which parts have bone underneath and which have fat/muscle.
Watch your colours on the eyebrows as well - one's brown where the other is bluish! (not that it really stands out, it's just something I picked up on close observation )