Oh! And this was done in Photoshop. But you can do the same thing in Painter, too.
The actual image was made for Dungeon magazine, issue 148, I believe.
EDIT: Some additional notes:
*You can use this method with full scale illustrations, not just portaits. It can be a little tricky figuring out what colors to use in the Overlay layer, but it's still a good quick way to get your overall color scheme worked out and (most importantly) unified.
*You don't have to start with green. Any darkish color works well. And if you don't like it, you can always change it very easily. Think of the Multiply layer as just your underpainting. Whatever color you choose, it's going to look weird until you start painting on top and bringing out the midtones and highlights. But that rich underpainting will help make the colors on top pop out that much more and saves you some time as far as figuring out your midtones and shadows.
© Paizo Publishing, LLC
I'm new to art and trying out everything that sounds fun -- this seems an efficient way of working, so am excited to try it out next portrait! I really dig the style you've done this in. Right now I'm carving out of blocks of colour, with no lineart and no b/w stage, instead a 'value check' colour layer, and multiply/luminosity clipped if I have to. It's direct and malleable, but I also miss the effect that an underpainting lends -- this could be a nice alternative!