To understand the relationships between colors, they are typically placed on a color wheel or color circle, with sections shown in different colors, and with the colors arranged in the order in which they appear in the visible spectrum. Color wheels are theoretical diagrams of the structure of the visible spectrum, or some part of it, and the relationships of one color to another. Another way to think of the color wheel is to picture a thin strip of the visible spectrum that contains the colors, and bend it into a circle to form a wheel.

Color wheels can contain varying numbers of colors, but usually contain a number divisible by three, to show the relationship between three colors and their intermediaries. They can also be constructed using various color models, but the simplest way to demonstrate a color wheel is by displaying the colors that most people associate with the rainbow, and explaining the relationships between them. These colors are usually thought to be six colors typically called red, orange, yellow, green, blue, and violet (sometimes called purple).

The color wheel shown above displays three primary colors typically used in simple demonstrations of paint combinations: red, yellow, and blue. This color wheel also includes secondary colors between the primary colors: orange, green, and violet.

Most people in the US consider these six colors to be the basic primary and secondary colors, but in practice, science and industry use different primary and secondary colors based on what products they produce, and what color models they require. Module 4 in this program will discuss more scientific and industry based color models.

If we include colors between all the primary and secondary colors on this color wheel, we have a wheel now containing 12 colors. The additional colors between the primary and secondary colors are known as tertiary colors, and include such colors as red/orange, yellow/green, and blue/violet.

Larger color wheels are used as well, such as a 24-color wheel containing intermediary colors red/red/orange, green/yellow/green, blue/violet/blue, and others. There is theoretically no limit to the number of color combinations that can be displayed in a color wheel, depending on the variations of color desired. Color wheels such as these are used for reference purposes in design, because our ability to match colors from memory is much weaker than our ability to match colors from a visual gradation system.