Modding Project: Ombre All The Hairs

I got a crazy idea to ombre all the non-ombre'd hairs from all packs and base game. I just don't know how to do it!  I couldn't find a tutorial that explains how people are matching their ombre recolors to EA's.

I'm thinking this is a large undertaking so if anyone wants in, please let me know. If anyone wants to point me in the direction of a tutorial or explain it to me, I would greatly appreciate it!

Thanks!