I am guilty of using all the colors in one painting and wondering why it looks so "all-over-the-place". So lately I am more and more interested in limited palette for illustration or concept art.
To show how it would work, I made a little sketch, from this photograph.
I tried to keep it simple by using only 4 values in the beginning, like Loomis wrote in "Creative illustration". White, light grey, dark grey and black. I also simplified a lot of the shapes into their essence.
Then I used the gamut-mask. It is easy to use. You simply choose the shape of the mask and take the colors that are inside the shape. I took a triangular shape, creating a triad-harmony. After picking the 3 colors from the cornes, I created 2 shades (dark version of a color) and 2 tints (light version of a color) by using the "Color and Saturation" adjustment in Photoshop.
I then painted the pre-made sketch, using clipping masks in Photoshop. That way I don't have to worry about painting outside the shapes, I designed during the sketch-phase. I tried to stay true to the warm / cool play of the colors, having my warmest colors in the foreground, using cooler colors for the background.
I did this 3 times. Everytime I chose 3 different colors. The final color-sketches all look different, having their own mood. But all are harmonious.
So next time you are having trouble, deciding on the colors for your illustration, give the gamut-mask a try.
I hope it helps :)