In order to account for lighter and darker shades of color, we now add light into the mixture. Artists sometimes say that "white is the presence of light, and black is the absence of light."

This image shows pure red, orange, yellow, green, blue, and purple, as they exist in the center band of the color wheel. The color wheel accounts for the need to create shades of these colors by adding light to colors as they extend outward from the center band of the color wheel, and removing light from colors as the extend inward toward the center of the color wheel.

The point at dead center inside the color wheel, the point where light no longer exists, is pure darkness or pure black. The area outside the confines of the color wheel, where light drowns out all color, is pure lightness or pure white.

The end result of this combination of 3 primary colors, 3 secondary colors, and the presence or absence of light, is represented in the color wheel. Your computer uses what is essentially a mathematical version of the color wheel to create the 24 million colors we see on a common "true-color" monitor.

Of course, a color wheel is just a representation of color. Being visual creatures, we humans tend to understand visual representations much easier than we understand complex mathematical equations. This is why the color wheel concept is used to explain how just three colors and pure white light can combine to create the millions of colors we see can on a typical monitor, and the infinite variety of colors found in nature.