Comment 1 for bug 323160

This also affects netpbm. Try for example,
    ppmlabel -colour green "label" somefile.ppm
and get output
   ppmlabel: can't open color names database file named /usr/lib/X11/rgb.txt, /etc/X11/rgb.txt, or /usr/X11R6/lib/X11/rgb.txt and Environment variable RGBDEF not set. Set RGBDEF to the pathname of your rgb.txt file or don't use color names.

I suspect there may be other affected programs too. x11-common installs a dangling symlink to /etc/X11/rgb.txt, but according to the changelog in /usr/share/xorg/changelog.gz the UBUNTU maintainers removed /etc/X11/rgb.txt because the x server itself no longer needs it.