I'm always thinking about palettes and color selection. Limiting colors creates better harmony in the painting, and it really forces you to rethink color relationships.
One of my favorite limited palettes to work with is a Zorn palette, made popular by Anders Zorn. Zorn primarily used yellow ochre, vermilion, and black. James Gurney has a great blog post with lots of information about him if you'd like to read further.
"Saving for Christmas" 12x12 inches (2012)
The piece above was painted with a Zorn palette. I used titanium white, yellow ochre, cadmium red light (scarlet), and ivory black. Notice the small spots of cool, reflected light in the warm side of the piggy bank. It looks blue, but it's really just a mixture of ivory black and white. Because color is relative to its surroundings, grey looks bluer because it is surrounded by its compliment (orange).
Relationships between colors are a very important part of my work. I love exploring the way colors interact and play off each other. Restricting oneself to the limited colors of a Zorn palette is a great way of testing these relationships, and making the "most" out of each pigment. Color shifts have to become so much more subtle, because you have such a smaller scale to work from. Also, when the range of color is so small, an emphasis is placed even more so on value relationships.
Zorn's palette is a very useful tool in learning to appreciate these relationships, and learning how to control them when working with a full-spectrum palette.
Here is a 12x16 inch Zorn Palette color chart.