At the end of season 2, Kaneki's hair turns back to its original colour.
I know his hair turned to white because of the Marie Antoinette (MA) syndrome, but in all the research I've done, I've found nothing regarding a cure for the MA syndrome.
So, why did his hair change back? Can the MA syndrome be cured? And why did Kaneki get cured out of the blue for no reason at all?
My interpretation was that it was purely symbolic. I saw the white hair as a change in his personality, a sort of draining of his "color" or original personality. When his hair changes back, we see some of his old self coming back. Keep in mind that writers don't necessarily make those sorts of decisions with medical backing. Pure symbolism is often a very large factor.
There has been several instances in the real human world where the hair of a person turns white overnight due to extreme shock and stress.
For evidences, look here and here.
Some well known rumors are Marie Antoinette and Captain Moody.
Why did Kaneki's hair turn white in the first place?
13th Ward's Jason tortured Ken Kaneki after capturing as wiki says:
Yamori then tortured Kaneki for ten days. He injected Kaneki with Rc suppressants to suppress his ghoul powers and cut off his fingers and toes like nails. When the injection's effects wore off, he forced Kaneki to eat so his fingers and toes would regrow. Later, he also placed a Chinese red-headed centipede in Kaneki's ear.
Such a magnitude of torture must trigger the autoimmune mechanism of Kaneki's body due to the immense stress on his body and mind.
Why did Kaneki's hair color revert back to black?
The syndrome is thought to be a variant of Alopecia areata. In nonscarring alopecia, where the hair shafts are gone but the hair follicles are preserved, making this type of alopecia reversible. I guess the human body could start attacking melanocytes too under extreme stress or melanocytes can modify pigmentation.
If interested, look here.
THE THEORY:
Due to stress and shock imposed on Kaneki's body by Jason over the period of 10 days, Kaneki's black hair starts to fall out at once. They are replaced by white hairs quickly (due to high regeneration power of Kaneki).
Upon finding his real self (on his meeting with Tsukiyama), the stress is finally relieved on his psyche and his turns black quickly due to melanin release. (His body stopped attacking melanocytes or melanocytes stopped modifying pigmentation.)
I see the changing of his hair color as a physical side effect of him forsaking his mother's nature (his nature), and if you will allow me to speculate, the white hair is symbolic of him embracing his father's nature, not that I think he knows it.
I'm just guessing, but I think Arima (the white hair guy fighting the big Owl, the man Kaneki runs into at the end) is really his father.
They go out of the way in episode 9 of season 2 to show Arima thinking long and hard on what to write in his will, but in the end he leaves it blank. I think it would be messed up to write your child into a will when that child thinks you are dead like, "Dear Kaneki, so I'm not dead, but if you're reading this, then I am dead, so ya, sorry. P.S. Love ya". But the desire to reach out would at that moment be strong. With Arima leaving it blank, he is showing what kind of man he is. A man willing to make the hard choices. The same (more or less) choices Kaneki is making.
At the end, he isn't able to walk away from his friend's request and willingly put himself in harm's way when at that moment no one needed his protection.
That's why I think we see his hair change over and over in the end... a battle between his two natures.
In the manga, Kaneki's hair slowly starts to turn black in Tokyo Ghoul: re, with Haise Sasaki's hair being white with black at the crown. During his battle with Kanae and then The One Eyed Owl, his hair reverts completely back to its original black colour. Though it takes a lot longer than the anime, this is the same kind of complete cycle symbolism described above.
I agree with what people have said about why it turns white, but I think the reason it turned black at the end was symbolism for him touching back to humanity through Hide, especially the last shot where we see Kaneki's hair go black and then Arima's white hair right after.
I thought it was kind of symbolic of the irony of how ghoul's are supposedly evil (in human's eyes) but Kaneki is showing how "human" he is when he carries Hide in front of all the CCG, whereas Arima is a human but coldly walks out front to kill Kaneki.
This is just my opinion based off the impression I got.
I think the change from black to white was physical as well as symbolic. It showed the change in his personality, that to survive the torture from Jason he had to leave his humanity behind and be a soulless ghoul. The symbolic change of his hair colour from white to black was to show that he had regained his humanity, the death of Hide brought about his emotions and his old self and old life.
From chapter 99 of the :re manga, Touka was talking to Kaneki and asked why he had that "sesame-pudding" haircut which apparently means black in Japan. He said that Dr. Shiba told him suppressing his RC cell activity increased his melanin production
Melanin gives you the color of your hair and skin, the production of this stops you from having white hair. That is why older people who have white hair have white hair. They don't produce as much melanin as they use to.
That explains why his hair turned black: he was suppressing his RC Cells. He fought Arima all out and so he had to use a lot of RC cells so his RC Cells weren't suppressed which gave him full white hair again.
Since Kaneki's hair turns back to its original hair color, I think that means he's getting his personality. Notice when his hair turns white he is completely insane, but when it turns back he gets his old self again.
In season 3's trailer or "release", Kaneki was shown with black and white hair. This means Kaneki has a split personality but not with the girl that was in control before but with his ghoul and human personalities. Since these are mixed and he is a half-breed, sometimes the ghoul personality is in control and his human personality is in control. Notice when he first becomes a ghoul, he says no to eat human meat but the ghoul side wanted to. That means due to the stress he had before and then the torture he went through much more, the ghoul side took over.
So I think somewhat the human Kaneki was fighting his way past the ghoul because he wanted to take control and be his regular self. If you were being controlled, I think you would fight to take control. That's what he was doing. In season 3, he will be in control but the thing is that the ghoul is still in his personality and the instincts will come back and he will have a split personality that will be stronger.
I feel that it can't be symbolic of him returning to his normal self. I mean, the reason his hair turned white was due to him being damaged... Right? Like the torture and all caused his hair colour to change, and Hide's death only adds to that damage. I'm pretty clueless
I think his change from white to black over and over again is because of his human side fighting his ghoul side. When he got tortured by Jason he accepted his ghoul side, but when Hide died it brought out his human side and his body doesn't know what to choose.
