Because I was having fun with this one, and it was an exercise, I expanded the canvas to see where things would go. Planning and sketches is great, but sometimes those doodles can turn into something. Below is a progress shot, without the adjustment layers.
If you're curious as to how big I'm working - Below is a 100 % crop of the face.
If it's for web or a Maya background...I usually work double of the final resolution. If you go to Window > Documents > New Window ... that will bring up an "instanced" (to use Maya terms) window. Set one window to 50% to see what it looks like from far away, and paint in the other window set to 100%. Hope that makes sense. Also, if you don't know this tip already - The keys [and ] on the keyboard change your brush size. Hitting 1 - 9 on the right side of your keyboard changes the opacity of your brush from 10 to 90%, with 0 making it 100%. Holding Alt gets you a quick color picker. B is brush, E is eraser. H is the hand tool, for moving around the big document. As with any program, knowing a few shortcuts gets you concentrating more on the art stuff than breaking your flow with menus and buttons.
Below is a shot of the hand, rendered again in monochrome...later adding the fun adjustments. I tend to paint with low saturations for some reason. Maybe because I mostly paint at night, or in a dark lab, so the contrast seems more intense? I don't know heh. I didn't even think to fix it till later...
Another note on layers. One of the goals of this piece was to keep it as "painterly" as possible. One way to force me into doing this was keeping all the paint on one layer. Color adjustments don't count. Now, as you'll see soon, it's good to have layers, when blocking in an environment. Also, if you're not sure about how something will look, add a layer, and paint it to see. If I like what I just did, I'll Merge all the Normal layers back together, and continue. This helps keep things organized and unified....because as you might know, layers can get into the hundreds, heh.
With the color layers, you're trying not to use these as crutches. You can actually use an approach of painting in purely black in white....then adding color layers and making a piece out of it (like the dragon on my site under Progressions) As others might say, and I, from practice believe too...that shooting for the right colors at the start is much better for you in learning how light affects objects and their colors. A guessed at what the green vest would be in light, and shadow....and found out later, adding some low blues, and reds in there do help it out. So that's what these layers are there for. Try getting it right the first time, but use the tools to adjust and elaborate your painting.