Color wheel
From The Glossary of Digital Photography
A geometric arrangement of colors that make their relationships easier to understand or visualize. For example, a color wheel can have 12 colors. First there are red, green, and blue (RGB) on the wheel, dividing it into thirds. Opposite them are their opposites, or complementary colors: cyan, magenta, and yellow (CMY). In between these colors are the colors that are mixtures of the ones on each side.
For example, the color in between red and yellow is orange.
