Hi Tony,I have to agree with Mike and melissa concerning the tonal values going on with the face. If you find drawing faces a bit daunting It can be all too easy to get to a point where you are quite statisfied with what you have already achieved and dare not push it any further in case you ruin it. It may be a good idea, just as an excercise, to do the same drawing again but this time force yourself to put in the darker tones that are missing from the original drawing and see what a difference it makes.The reason I say do the same one again is purely because as you have done it once, you should'nt feel the pressure to "get it right" and settle on "almost right" doing it a second time.
If the original source photo is a bit light use photo shop, or whatever, to manipulate it a bit. I have just started a double portrait (finally!!!) and have four separate prints of the same image to give me a range of depth for various bits (1) for skin, (2) for hair (the origianl its too light, almost non existant) (3) for denim (to bring out the grain) and (4) to show up the weave and pattern a bit more in a knitted cardigan.
Work the eyes, nose and mouth first getting in those dark values and then you HAVE to get the other stuff to balance. (This next bit should answer your question) Don't make life difficult for yourself, ALWAYS work from a black and white reference to get those tones spot on, its all too easy to go off course when working from colour reference only. If you are going for black and white photo-realism..., use a black and white photo....simple
Hope this helps some.
Phil
www.graphite-art.com