Suppression of RC activity increased his melanin activity
Increased melanin = darker hair. Now it makes sense why his hair turned all black when he regained his memories. Haise had “pudding” hair because all around, he was confused about his own identity. To supress? To not supress? A human? A ghoul? Who exactly was he before? But after he regained his memories, he knew exactly who he was, and fitting the role of a “ghoul investigator,” Kaneki decided to suppress his RC cells. He also consumed Eto’s kakuho after his fight with her,
and he gained a mountain of RC cells. Suppressing it must have been incredibly difficult, which kept his hair constantly black
But now he has stopped the suppression, and his hair has turned white. And with it, other qualities show as well…
Kaneki usually has to put a lot of strain on himself, mentally and physically, to assume his kakuja. But even in a relaxed state, his kakuja form has manifested in his hands. Kaneki isn’t holding back anymore, but even in his normal state his RC cells manifest. He real strong. Who knows what his RC level is now. But Kaneki isn’t even holding back his kakugan anymore, or can he even hold it back now that he isn’t suppressing his RC cells? Is holding back proving to be too strenuous and difficult because he’s so powerful?
Kaneki couldn’t control his kakugan at the beginning of the original manga, but that might’ve been because he had no control over his RC cells. That makes sense because his hair was black (not because he was suppressing his RC cells, but rather that there weren’t even enough RC cells to suppress). And after Yamori strengthened Kaneki’s RC vessels through continuous torture and regeneration, Kaneki’s hair turned white after he let everything go. He wanted to be strong. With the concept of suppression, the strong link between his hair and mental state make sense. But this dilemma brings a certain character to mind…
Does Uta also have overwhelming power? His black hair could be from suppression, meaning that despite that his kakugan still shows because he’s so powerful? But it could also just be hair dye. Who knows, maybe he just likes showing his kakugan off. Or maybe this RC suppression deal only applies to half-ghouls. After all, Seidou also had the same experience and white-hair transformation that Kaneki had.
Imagine this: Love Never Dies AU in which Christine realises what her priorities should be and leaves Coney Island with Gustave only, because she’s had enough with silly men pretending they own her and her son.