I have this theory:
His hair is originally white, but to keep his personal life private he dye it black. Because he spent lot of time in the rift, he return to white because there was no way he could dye it inside the rift.
Now, according to the novel (some users write in other comments), seems like he is going to return to white
His hair is originally brunette/brown as it’s shown at the start when he was active as J. Since he spent like 8 years in the rift, his hair lost the pigment and turned white, and he dyes it now.
His hair is originally brown black as shown in the chap 56. His white hair is a part of the plot thats will be explained later
Well...seems like my theory was wrong
Thanks for correct it
I’m a little late on saying this but in chapter two he has white hair and if I’m not wrong I swear in another earlier chapter he talked about dying his hair