How to change the GDM default background wallpaper to my customized wallpaper or solid color?

Question

I was trying to change the Ubuntu 11.04 GDM default background wallpaper to solid color such as black. However it's not working; here is what I've tried so far:

1. From the terminal:

   xsetroot -bg black -solid black 
   

2. From /etc/gdm/custom.conf:

   [greeter]
   BackgroundImage=/var/tmp/x1.png
   BackgroundColor=#000000
   BackgroundType=2
   

How can I make it work?