/usr/share/gmt/cpt/rainbow.cpt is in gmt-common 5.3.1+dfsg-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | # $Id: rainbow.cpt 16562 2016-06-18 04:04:08Z pwessel $
#
# Standard rainbow colortable for hue = 300 (magenta) to 0 (red)
#
#----------------------------------------------------------
# COLOR_MODEL = hsv
#----------------------------------------------------------
0 magenta 1 red
#----------------------------------------------------------
|