It ranges, really, from 'quite simple' to 'a challenge but fun once you get it' to 'why did you even Maxis what the HELL' in terms of how hard base game hairs are to recolor. AlfredAskew thinks they had some kind of program that allowed them to paint on the 3-D image of the mesh, not just the flat texture.

Sometimes all it takes is a little bit, though!