To change the splash screen of Gnome, you need to do the following.

1. Download your favourite splash screen and store it in some location.
2. Invoke the GConf editor (Applications --> System Tools --> Configuration Editor).
3. Change the key for the splash screen, /apps/gnome-session/options to store the absolute path of the splash screen that you want to use.

To change the background color while the splash screen shows, do the following.

1. Invoke the GDM configuration dialog (System --> Administration --> Login Screen Setup).
2. Change the background screen color to the desired color that suits your theme.