（￣へ￣）hmmm.. i think he dyed his hair green/blue to look similar to his family but his true hair colour is black (▰˘◡˘▰)
Since childhood? It makes more sense he died his hair after being kicked out so people don't suspect he is related to his Household. He was trying to pass as a commoner afterall.
So... I was rereading the thing and you are actually right. Chapter 18 says Hardy always had black hair but most of his family has turquoise hair and they wanted him to match them so he changed it with magic. Once kicked out he changed his hair back. So yeah, it was since childhood weirdly enough. That explains the AU as well because he would have kept his hair turquoise if he didn't get kick out by his family.
Guys if you go back to chapter 5 page 5 you can see the flashback of them as kids... I think Hardy's original hair colour was green blue?? Did he dye his hair??? Tbh I have no idea this is just a guess lol
But anyway they could've given us a warning that it was an AU... Why'd they spring it like that I was so scared