1. Click Start > click Run > type regedit > and click OK.

2. Navigate to the following registry key:
HKEY_USERS\DEFAULT\Control Panel\Colors

3. In the details pane (right-side), double-click the Background string value item.

4. Change the value to a 3 number RGB value, and then click OK .

You have now changed the logon background color.

NOTE: I went looking for this registry key after using the "Change the Logon BackGround" tweak, and that bright XP blue color kept showing before the OS had a chance to load my dark wallpaper. This way if you choose a color close to the wallpaper shown, the load of the wallpaper is less noticeable.