Well, I am glad you posted your ?.
I checked your work. It needs to have more color, deeper tone. It looks good though.
There are tons of books by North Light books
http://www.artistsnetwork.com/nlbooks/
I know they have stuff on skin tone (ppl in all kinds of mediums).
Also you might want to join the NL club. It is cheep and I have had a good experience. If you don't want to do that review books at Barnes&Noble and then get the books from Amazon or half.com.
Trust me Amazon is a better value than B&N on most things.
Another option to learn more is to go to your library. I know they have books on rendering people. Many of the NL books are at libraries.
Finally, I suggest posting your work at
www.Artpapa.com, in the
graphite and color forums. I say graphite because that is more active than the color sub-forum. Just let them know you are wanting to learn skintone. You can also post and ask from the pros at
www.wetcanvas.com. Artpapa is a very nice forum and smaller than wetcanvas.
A.