The IRP mod tends to use local lists within the mod for colors as the global lists for colors did not exist and the author did not feel like creating code to search for them in races.gd. Therefore if you want to add new colors it will have to be in portrait.gd of the mod. However, the way the mod is coded, I do not believe it ever checks fur color as everyone has a hair color and that overrides fur color. I have not tested it, but it appears that changing in randomportraits.gd:
elif person.furcolor != null && !person.furcolor.empty():
to
if person.furcolor != null && !person.furcolor.empty():
should cause fur color to override hair color if it exists. Unfortunately, with the way that it is setup it would take significant changes to support both.