Comment 31 for bug 300935

Hm, none of these are core applications. However, the impact on them being non-core applications is pretty severe - because you can't use them anymore! So this should call for a Medium importance.
Unless you count the relatively simple workaround of downloading a rgb.txt from somewhere (or take it from another package, see below), and copy it to /etc/X11.

Other applications affected by the bug:
xcolorsel

Note that "netpbm" is not an application in itself, but a package consisting of over 200(!) utilities to convert and manipulate PNM images. Although admittedly only a few(?) of those utilities broke because of the missing rgb.txt.

Several packages have already included a (redundant) copy of rgb.txt locally, since the file seems to have been moved through the filesystem several times in the past already:
emacs
latex2html
pgplot5
pixmap
xplanet
xwatch