I love the Gnomon stuff, plus I'm lucky in that I can usually get my boss to pay for it ;-)
I tend to prefer the ones that cover workflow and technical approaches, over the more how-to-draw stuff. A lot of drawing is practice, practice, practice, which unfortunately you can't get from a DVD :-)
They have one on coloring (link below) that spends a good amount discussing setting up the file, working with aliased friskets, color blocking, GSR, etc. While there's a lot of basic coloring techniques in there, its real value for me was more how he was doing it, rather than what he was doing. If that makes any sense.
http://www.thegnomonworkshop.com/store/product/297/Comic-Book-Coloring-with-Steve-Firchow