This month's exercise was to make two color wheels, one warm and one cool. Warm colors usually have some red in them. Cold colors usually have some blue in them.
A color wheel is more then just the rainbow of colors in a circle. It helps you envision how the colors will work together (or not). Colors that work together well are either analogous or complements.
The most intense secondary colors come from mixing two primary colors that contain a bit of the secondary color already. True greens lack any hit of red, which means they are made from cool yellow and blue pigments. Likewise, bright oranges are made from warm yellows and reds. True purples do not have any yellow, which means they are made from cool reds and warm blues. These mixes are displayed in a split primary color wheel.
Limiting the number of pigments that are combined to three or less is best, because as more pigments are combined the color becomes duller and duller. Mixing colors from the opposite sides of the color wheel (i.e. orange and blue) will create a muddy grayish color. Although, these colors have their place in the painting too.
As an interesting side note, we owe our color wheels to Isaac Newton, who did numerous experiments with light and color eventually creating the first geometric model of color perception, the 'Hue Circle'. While researching color wheels I found the "Color Sphere" proposed by Philipp Otto Runge. Newton's 'Hue Circle' circumnavigates the equator, and a white to black value scale that runs north to south is superimposed on it.