i think his original hair is grayish or white but since he's masking his identity, he colors it black ? i might be wrong. he uses a mask that hides his face and distorts his voice so he won't be recognized so he probably colors his hair for that same reason too since there's a chapter of him recoloring his hair to black.
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So far I've only seen 2 manhwas that has th is type of condition
When u been in a rift for a LONG period of time "corruption" starts forming and one symptoms is ur hair turning white which was in euijae case if I'm not mistaken later on in the novel it will be addressed and there was one character (also important character but I forgot HIS name yes I vaguely remember it was a guy or guys) who had the same condition as euijae and turned into "monsters" mutation basically
There was also a part in the novel specifically around 265 where euijae was dying in a dungeon(the 2nd timeline) bc of that mutation bc its either he chooses to die or will mutation :))
Thats also the part where sayoung will get left behind in that "dungeon" (i forgot the term of it kinda like a memory) and euijae would have to WAIT for him while also actively trying to get him out

Did I missed something? I thought I saw on early chapter J's hair back then was grey or white?