Just thought of another tip for working with color. This is something I've been trying lately, and the method really helps me.

If your using photoshop, create a new adjustment layer, on top of your color drawing. Set this adjustment layer to 'Hue and Saturation'. Turn the saturation down all the way.

Now, you have a layer that works as a color/grayscale switch, to use for checking your color scheme. Turn it off and on, as you work with the color.

The most important part of painting, is having the correct values of light and dark. These define the basic shapes. The color scheme can be anything, and if the values of the color are correct, it will still stand up. The color saturation is affected mainly by the distance the viewer is to the object. A yellow ball in full light up close, will be a very saturated, rich, yellow. The same ball in the same light 50 yards away will be a more desaturated (closer to grayscale) yellow.

Not really trying to get into 'color theory 101', but I tend to geek out on this stuff. I'm sure you have heard this stuff before, but I struggle with color tone/saturation all the time, and having it broken down like this really helps.