If using gdm as login manager, try Menubar>System>Preferences>Desktop Background then change the background colour to whatever you prefer, this won't affect the desktop wallpaper.
If using kdm as login manager try kcontrol (in terminal) then
System Administration>Login Manager>Background.
I'm not 100% certain that will work but it ought to.
Last edited by fannymites; 01-15-2006 at 07:06 AM.