I know absolutely nothing about Photoshop. I've had it a grand total of... four days? :lol: And I wanted to start off slowly, so all I've been making so far is icons. But I guess that since I've been learning, I can at least tell you, as a beginner, what I worked on.
You should experiment A LOT with layers if you haven't already. They help a lot with coloring as far as I can tell with my very limited experience. Try duplicating your image into several layers, playing around with the blending options (normal, soft light, screen, multiply, etc), and seeing how different the image looks by rearranging those layers.
To really play around with color, go to "Create new fill or adjustment layer" in the Layers/Channels/Paths window (it's the button that looks like a black and white cookie), and select "Curves..." Also, regarding colors, under the "Create new fill or adjustment layer" you can select "Selective Color..." and play around with all those options. Under all the colors, try changing the cyan/magenta/yellow/black values and seeing how they affect the image. You can create multiple of these layers, so you don't have to try to get the colors the exact way you want them in one "Selective Color" layer. Actually, that pretty much goes for all layers.
By the way, if you want to make yourself sets, knowing how to crop and resize images is useful knowledge.
I'm not very fit to give advice, but since you only have a limited amount of time, I hope you can make something from my advice. ^_^ Here are some samples of icons I've made so far experimenting like this... (Warning: lots of Jam :wub: )
Jenna & JohnMe!Jenna & John2Natalie PortmanV for VendettaPam CryingPam Crying2Pam Crying3Audrey HepburnJim & PamJim & Pam2