Color is the emotion of art. Sharpness of shade can change an entire mood. Choose carefully the basic initial colors your going to use in an environment. Use these colors to construct the whole scene. The mood you get from the key will carry throughout the entire work.
Construct a key by choosing related, and unrelated colors. Complementary pairs will energize the work, while analogous will extend the hue you're working with. Pair yellows with oranges for a simple analogous pairing.
The key will determine the depth of shadow and cast of light in all the objects and subject matter in a work. If you have a figure dressed in blue, but there is no blue in the key, choose the color closest to blue and use that. You can also add a little blue to the most similar color to go outside the key. Stick to the key as close as you can to keep the mood you've established.
Edit and refine your key as necessary.