How to force background colour with GPO centred wallpaper?

The background image is an option already User->Policies->Admin Templates->Desktop->Desktop->Desktop Wallpaper.

The desktop color is not however, you'll need to create an ADM file an import it. Here's the file I use:

CLASS USER
 CATEGORY "Desktop"
  CATEGORY "Custom"
   KEYNAME "Control Panel\Colors"
   POLICY "Background color"
    EXPLAIN "Allow you to control the background color of the user's desktop."
    PART "Enter the RGB values of the user's desktop background (ex: 128 128 128)" TEXT
    END PART
    PART "Color" EDITTEXT
     REQUIRED
     VALUENAME "Background"
    END PART
   END POLICY
  END CATEGORY
 END CATEGORY

Open notepad, pop that in, save it as something reasonable with the file extension .ADM

Open the GPO, right click Admin Templates, Add, Add..., Find the file, Close. Find the setting in User->Admin Templates->Classic Admin Temp(if on Vista or Win7)->Desktop->Custom->Background Color

Note: Since this GPO is processed after the user logs in, and after Windows sets the background color, the first time the user logs in they will default to whatever the machine's default background color is. I've never been motivated enough to write the ADM file to set the default background color, though it should be possible. In any case, the second login and forward the color will be set correctly. This is due to how MS implemented the background color setting in the registry. Also, after this GPO is removed the user will retain the color, it will not revert as with most GPO settings; again it's how MS chose to implement the setting...