I've never really looked into changing its color, but you can disable it and force a black screen, which IMHO looks a lot better.
Edit KDM's Xservers file which on LFS is in /opt/kde/share/config/kdm/Xservers and look for line like this ...
:0 local@tty1 /usr/X11R6/bin/X
and change it thusly
:0 local@tty1 /usr/X11R6/bin/X -br
Now when X is started by kdm you'll briefly get a black screen with a cursor till the login window appears.