Summarize the following:
". Your Mac has a palette of millions of colors for you to choose from. A preview of the color appears in the box at the top of the window. Let go of the mouse button and your desktop will change to your chosen color. You can do this as many times as you like, until you find the perfect color.

Summary:
To choose a color of your own, click on "Custom Color... Hold your mouse button down and move the pointer over the color wheel.