Ishida-sensei often uses hair color changes to visually illustrate a characters change in perspective/personality.
Juuzou prob dyed his hair.
Kaneki’s hair turned white from stress after he was tortured by Yamori. Some people call it, Marie Antoinette Syndrome.
Marie Antoinette Syndrome is a sudden whitening of the hair. The event that named the syndrome was the observation that the hair of Queen Marie Antoinette of Franceturned stark white after her capture after the ill-fated Flight to Varennes during the French Revolution. Witnesses have alleged that Antoinette’s hair suddenly turned white on three separate occasions.
The syndrome is thought to be a variant of alopecia areata diffusa or autoimmune non-scarring hair loss that affects selectively all pigmented hairs, leaving only the white hair behind.
Triggers activating the autoimmune mechanisms have been postulated, including sorrow and fear, but also fits of rage, extreme amounts of stress, unwelcome, unexpected news and overindulgence in sexual activities.