When you separate visible light into its component colors with a prism, most people will be able to see at least six colors. Those colors are red, orange, yellow, green, blue, and violet.
Red, yellow, and blue are the three primary colors. Pairs of these colors can be mixed to form the secondary colors, orange, green, and violet. If you mix a secondary color with one of the primaries that make it up, you get the six tertiary colors. For example, two tertiary colors are red-orange and yellow-orange.
Each color has only one complement. Isaac Newton defined complements as two colors that, when mixed together, result in gray. Depending on the pigments used, however, the result can be any "neutral" color from gray to brown.
The color wheel is a circle containing at least the six primary, three secondary, and six tertiary colors in the order in which they appear in the visible spectrum. Some color wheels will have even more. A color and its complement are always directly across from each other on the color wheel.
When designing a room in your home, or choosing an outfit for a special occasion, or even trying to come up with attractive gift wrapping, it is helpful to know something about color harmony. Color harmony is a way to choose related colors to create a unified or harmonious look.