DOKK / manpages / debian 12 / x11-xserver-utils / showrgb.1.en
SHOWRGB(1) General Commands Manual SHOWRGB(1)

showrgb - display an rgb color-name database

showrgb [ database ]

The showrgb program reads an rgb color-name database from a text file and converts it back to source form, printing the result to standard output. The default database is the one that X was built with, and may be overridden on the command line. Specify the database name without the .txt, .pag or .dir suffix.

/usr/share/X11/rgb
default database.
rgb 1.0.6 X Version 11