To the best of my knowledge there are three places where you have to change the color.
First is the vertex color in the hair mesh that defines the base color. You have to change this in order to get some reasonable effect at all.
Second is the specular color in the xml that is exported along with the mesh, so the reflections are in line with the new base color.
And third is Triss' head texture. It doesn't affect the hair color itself, but the light red always seems to find its way through the hair mesh
I tried this as a proof of concept a few days ago with Yen's hair mesh (colored to red) and put it on Triss. Looked weird but worked...