There is a http://www.gnome-look.org/
where you can find a lot of GDM themes.Then when you've downloaded the package open Login Window>Local and click on add
new GDM them,then just select the package and click install
.Select new theme and that's it.
If you have universe repository enabled then you can do from the command line;
sudo apt-get install gdm-themes
and after that you'll have more GDM themes to choose from.There is a background colour button also for GDM in the Local menu,just click on it and select the colour you want.