Using this option you can select a background color for all of your windows, and a background color for your listboxes.
The colors can be set fix by the template, so the user cannot change them, or controlled by variables, so you or your users can change them at will.
Background color have a great advantage over wallpapers: raw speed. Drawing a window is way faster using a plain color than a wallpaper, and the difference is notorious if you use any sort of remote control program (like Terminal Services or Citrix).
A colored window may look like this:
