It's sort of complicated on why he makes you dye your hair black. He believes people will less-easily recognize any similarities between you two if you look like the less recognizable version of him: Dabi.

Everyone remembers when he dumped that highly-acidic concoction on his hair, washing away the black, when he broadcasted for the world who he really is. They remember Endeavor's fuck up, Toya. So he thinks it's better if you look further from 'Toya' and closer to 'Dabi'.

"I said stay. Still." Your father huffed, pressing onto your shoulders to keep you on the stool. His hands were probably going to be stained for weeks because of his lack of gloves. He massaged the dye into your scalp, making sure to get all of your ends.

Not to brag, but like, he's kind of a pro at this. "Yeah yeah. Your roots were a whole different color the last time you did this yourself, so pipe down."