Why is the color wheel important?

The color wheel is the lifeline for any serious painter. It graphically shows you what colors to mix to get the effect you want. For example, we all know that the colors in the tubes or jars of paint are intense and gaudy if used as is. So we have to "grey" them or tone them down. This is done by adding it's opposite color, which will be directly opposite of it on the color wheel. The color wheel can also serves as a quick reference if you need a scheme of analagous colors, with maybe a bright contrast.