Hey Anon 🙂 

Ah, I just didn’t realize Kaneki’s fingernails became purple to be honest? Aren’t they simply black? In any case, if you’re talking about his different fingernails after the Aogiri arc in TG, that’s because of Yamori torturing him and his fingers and toes keeping on regenerating every time. 

As for the hair color changing so much, the explanation is simple really: it’s supposed to show a change (progression) in Kaneki’s character development each time a different level has been reached. 🙂

Basically this is a circle: Black (TG vol 1-6) -> White (TG 7-14) -> Black & White (as Sasaki Haise) -> Black again (until the Cochlea arc in :Re) -> White again.

We went from the innocent and rather naive boy whose motto was “instead of a person who hurts others become a person who gets hurt” to the boy who thought he had destroy his enemies in order to protect everyone…

to Haise the investigator who “didn’t want to disappear”…

…back to Kaneki with his memories again who decided he was to sacrifice himself for others (inwardly because he thought he wanted to die)…

to him finally realizing he wanted to live.

And I don’t know if you noticed but the difference in TG and :Re on that point always makes me smile because Kuroneki (black-haired Kaneki) in TG was the rather joyful one in comparison to Shironeki (white-haired Kaneki) while in :Re Kuroneki wanted to die while the current Shironeki finally realized he had to and wanted to live. 🙂

If you want to read more on the subject you have @sans-san’s very good post about hair color changing for Kaneki, Hide and Juuzou 🙂

I hope it answers your questions! 🙂 Have a nice day/evening Anon 🙂