CB you fucking rock my socks. In a good way. You probably have an idea of what you want this to look like, so I don't want to try and change your intent, but I'll give my two cents.
Right now you've got one color down, and I can't tell if you're going to make this monochromatic or if that's just a base color. If you go for monochrome, that will probably look great for his expression. I originally wanted "Portrait of a Man" to be totally blue, but instead of painting it blue from the start, I painted it normally and then adjusted colors later and decided I didn't like it. You might consider doing that, it will give you a couple colorings to choose from, and if you go for monochrome, the tones and stuff will probably end up a lot more accurate.
If you're going to go a little crazy with the colors, though, you could try something I haven't really done much myself. Where you might have a dark, dark shadow, go ahead and throw in a little deep purple, and some light reds or neon blues to really bright spots. I know you're probably inclined to just paint the colors you see, but in reality the colors you see are just a big mixture of a whole bunch of other colors. Putting colors where you think they might not belong isn't necessarily such a bad idea. Like on a jaw with a hard shadow, a faint streak of purple next to (but a little distanced from) the edge of the jaw gives a great effect of volume. (I don't really have any examples to show you, but I think you might know what I'm talking about.)
(You can do some really cool stuff with color... I once saw a guy paint a portrait where every brush stroke was a different color, but when you converted the portrait to grayscale, it looked like a flawless black and white photograph... shit like that is just amazing...)
But really, just paint this however you want to. I'm just saying all this stuff because I'm excited someone else is actually doing something like this, that doesn't mean you have to listen to me. Go at your own pace, do what you want to do, make it look like what YOU want it to look like.