Answer:
The color wheel is used to show the relationship between colors. Complementary colors are across from each other. Triadic colors are the ones at the points of an equilateral triangle. The split complementaries of a color are the ones immediately to the left and right of the original color's complementary. Analgous colors are next to each other. Quaternary colors are those touched by the corners of a rectangle. All of these sets of colors work well together and create unique looks and moods to paintings. You can dull a color by adding its complement. Sound confusing? You can buy a nice pocket sized one from an art supply store that demonstrates all of this.