I read that its ash and stuff built up since he can't shower or wet his skin because of his skin grafts
Really? Where did you read that? Is it in the manga? Which chapter?
Probably fan theory
In the anime he has red hair. So either it actually is red or the ashes turned his hair like that.
It was a fan theory. They went in depth about how healing skin grafts can't get wet or they fall off/don't heal. You should read it they go into detail lol
The only shocking thing here is Dabi didn't dye his hair actually black but has white hair with red spikes. He waited years for his dramatic reveal. Every morning he would put on his black hairspray and be like "Today is the day!" he always carried a waterbottle with him for this moment LMAO