I think that is excellent, and thank God you haven't worked in white, as its a colour so easy to overdo. Regarding hair, hair tends to clump together try not to think of it as individual strands, but as black or brown waves, swirls, and points. Remember to reflect the light on the tops of the waves. You could work in some grey to the beard, as this would give the character (i presume shakespear) an experienced look. Indicate maybe some strands of hair, and the rest more or less solid. The eye will compensate after that. Don't do what I do, and that is to keep "fixing" until I ruin something ! But the portrait is really excellent =]