This is a method of painting computer graphics that I use. The goal is to show how to get reasonably good results without spending too much time. (The example is a title screen background for a centipede-style game I'm working on.)
It's a good idea to save each step, or major change to a separate file so you can go back if you mess up. As you go through this process, keep this rule in mind:
Darker areas are more transparent, highlights are more opaque.
First is the color palette. You don't need a big palette with a lot of colors. I use this 3-level RGB palette because it contains the colors I need:
Use a "Normal" blending mode for all but the final step.
Paint the background with light brown. Using dark red, outline your shapes, and perhaps block out really dark areas. The purpose of these colors is to tie the colors of all objects together so they appear to exist in the same environment together.
Color in the shadows using the dark red. Add highlights back in with the light brown if you need to.
Deepen the shadows with black. (This picture should have more shading but you get the idea.)
Start adding gray using a soft brush. Try to make the highlights almost opaque grey, while letting the dark areas show through more. The purpose of the gray is to simulate an indirect, cold light source.
Now add white to the highlights to make them brighter.
Now color can be added. The dark blue is used for shading, and warmer colors in the highlights.
At this point, concentrate on adding small details to the brightest highlights. Use an "Addition" blending mode as is will preserve the color intensity better.