Responses
It's always been black. His hair only turned white during his time in the rift. When he got out, it was still white, so he just dyed it his original hair color again. That's why flashbacks that occur before he stepped in the rift had him depicted with black hair.
Look at the first chapters of the manhwa too

Why was his hair white but now its not? I know he dyed it, but it has remained black? Does anyone know if they have explained this or showed this more? Thank you!