Hi Lord of the Weather and welcome to the forum.
Yes, it is possible to edit the color maps for the various variables.
You need to find where the /openGribs/XyGrib/data/colors folder is located on your system. On a current Windows machine it will most likely be under <user>\AppData\Roaming\.
In the colors folder there are text files for each weather variable. These files contain the color values.
Best to make a safety copy of a file before you edit it.
Each line starts with the meteo parameter value and then 3 RGB values for that range. Parameter values and corresponding RGB values can be edited.
Have fun
David