Welcome aboard, Bayat.
The gdmsetup utility, as you may have already discovered, will not help here. That will allow you to change the background of the GDM login screen itself, whereas I believe you are trying to change the background colour of the screen that appears after
you login but before
The method described in the thread you mentioned would seem to be your best option, even though it isn't Mandrake specific.
I have both a Mandrake 9.1 and a Debian installation on my system at the moment, and under these the files I would need to edit are:
In this file you should find a section as follows:
# Mandrake default background
if [ "`whoami`" = root ]; then
xsetroot -solid "#B20003"
# Load the color only when not called from gdm/kdm
if [ -z "$1" ]; then
xsetroot -solid "#21449C"
This will change the default background to B20003 (a deep red) for a ROOT login and to 21449C for a normal user login. These are the numbers you need to change - they are standard hex codes, just like you'd use for setting colours in a webpage. You will need to be root to edit this file.