Why did Kaneki's hair change color?
You should read the manga. His hair turned white because of the stress of being tortured 2 or 3 times everyday for 10+ days. 

The anime is wrong. Why? Because Kaneki's hair didn't change in a moment, in the manga they show us his hair gradually turning white while he also  loses his mind. 





 

They don't explain why the hair turns white or the nails black (also they are not black black, they're more like super dark blood colored), but we can assume that the hair it's because of the Marie Antoinette syndrome, and the nails are because of the constant regeneration. Like I said before the nails are like dark blood colored, they're not pure black, it's like when you hit a finger really hard, and the nail falls off in a time? Well before that, it gets really dark blood colored, Kaneki's are like that. 

I know you posted this a time ago but maybe if you can read it, or other people that has the same doubt will understand.