I know necromancy is bad but I think I should share some new knowledge (new to me at last) on the subject :
even if I have deleted the offending hair (responsible for the bald head being black) I still have 2 sims which have black bald hair, no matter what I do, or how many times I make them go to the mirror.
But this evening I had a new idea and it work : I try changing these sims' age (down to teen and back up to adult) and it worked !! Somehow they get updated and no more black bald head
I try another method to do it and it works as well : changing the sim to zombie then back again to normal sim. Personnaly I prefer this method (lesser risk to mess wants, lifetime want and no need to replan their outfit...).
Hope this could help someone with the same problem.
Pioupiou