On Hair Colours in Tokyo Ghoul
Warning: Brace for nitpicking.
Basically, I think most of the constantly changing and “unnatural” hair colours in TG are simply a mixture of symbolism and Ishida’s art stye, and shouldn’t be taken too literally.
Look at how much the “same” hair colour can change in Ishida’s art depending on the drawing, from blue to red to black to green:
(Almost all pictures below are from Zakki)
Like these are all white-haired Kaneki
Black haired Kaneki, from shades of brown to blue to greenish.
And white haired Juuzou from TG, with his hair looking practically lilac on the right
Eto, from green to an almost purple-ish grey colour
And TG Touka, who seems to have purple and black hair more often in Ishida’s drawings than the blue the anime gave her - I actually struggled to find a “blue haired” Touka drawing from Ishida (not counting :re), it’s mostly pretty much black/dark purple.
Touka and Black-haired Kaneki seem to usually have the same hair colour when Ishida draws them together - his hair actually often looks lighter, unlike how his was black and hers was blue in the anime.
I don’t think these characters are literally constantly dyeing the hair. I don’t think Touka literally has blue hair, or Eto literally has green hair, I think it’s mostly just Ishida’s art style and symbolic of their personalities or changes they go through.
Though there are a few exceptions, like Kaneki’s white hair, which characters comment on it changing to white in the manga.