Those portraits look great! That means you already have a handle on some of the trickiest drawing skills, proportion and placement. Keep it up!
While you're doing the opaque color studies Hannes (algenpfleger) recommended, try to notice and remember some general principles about how color behaves. Things like:
- Value is separate from color. (This is why our brains can interpret forms in black and white photographs or forms under crazy colored lights just as well as forms under plain sunlight.) Get your values right first, then worry about color temperature, hue, and saturation.
- Hue and saturation affect value. You can't just go to the HSB sliders in Photoshop, set the black level slider at 50%, and expect every hue to give you the same value. That's what I did until yesterday, but now I know better. -_- (See Sycra's hue/value guide in the Resources section here
- Color is relative. You don't always want to use blue to paint something blue. Find a painting or photo with orange predominating and color pick something that appears blue. That color will probably be a desaturated orangish hue that only seems blue because the surrounding oranges are more saturated. The light source shifts local color (what you'd see under white light), as though you're looking through tinted glass.
- Saturation (color richness) tends to decrease with brightness. Look at the highlight on a nose, for example, and you'll find it's a brighter and paler color than the surrounding midtones.
- Saturation affects color temperature. Desaturated warms look cooler, desaturated cools look warmer. Change saturation to tweak a color's temperature before changing the hue.
- The temperature of light is reversed in shadow. So cool light produces relatively warm shadows, and warm light produces relatively cool shadows.
- The darkest dark is often extremely saturated and warm, even in warm light.
- Muddy colors are the result of inappropriate color temperature.
Don't think of these as set-in-stone rules, but guidelines that hold true in most cases. And feel free to ignore my instructions. :P
If all this stuff seems overwhelming, just remember you only have to focus on one thing at a time. You can do studies on individual aspects of color and combine that knowledge later. Feel free to let me know if I said something confusing. James Gurney and Richard Schmid cover the above and more in their books, and probably explain everything more clearly than I did.
Congrats on that job, and enjoy your holiday!