On top of his great gallery he has a page dedicated to showing his digital painting process. Having taken an oil painting class and doing other color studies, his process makes a ton of sense.
He first takes his sketch and color balances the lines to a sepia tone. Then, with another layer on top does on "Underpainting" to setup is tonal values throughout the piece.
I'm not going to go through the entire process, but I encourage you to head over to the demo page and check it out for yourself.
edit: Removed the images because I read his copyright information. Sorry Bill, I hope there are no hard feelings.