Check in your /etc/X11/xorg.conf and look at the Files section. You should see an entry like this:
Code:
RgbPath "/usr/X11R6/lib/X11/rgb"
in this example I have the file /usr/X11R6/lib/X11/rgb.txt containing the definitions of colors. Check if the file specified in your xorg.conf exists and if not either copy the rgb.txt from where it actually resides or change the entry in xorg.conf. To look for that file in your system just use find
Code:
find / -name rgb.txt