Red hair is associated with a temper or an outgoing personality. That core idea exported into anime. Red heads are full of raw energy and often associated with fire. Red heads are resilient; they seek to survive at all costs.
Brown hair, in particular, tend to be the most widely used color for ordinary Japanese characters, including the main characters, because a black-haired female can indicate other qualities.
Purple. A color that signifies royalty, nobility, luxury, power, and ambition. And for many of us, it is the "evil" color. A color that is usually portrayed when evil characters power up.
Green hair represents the cycle of reproduction and change. Greens also tend to be rejuvenating characters: healers. Yellows are also rejuvenating characters, but they tend to focus on the emotions while greens heal the body.
Fittingly, characters with pink hair signify romance and innocence. Pink hair is usually reserved for female characters. They are daydreamers and idealistic, with a child-like innocence. In many cases, they bring positive energy to the people around them. However, male characters with pink hair can be perverse.
Orange is a mix of blonde and red character elements. Orange characters tend to be flamboyant and attention seekers. The color has the energy of red hair and sometimes either the goofy, naive, or dumb aspects of blonde.
With elder characters, white hair denotes maturity, wisdom, and dignity.
Such a character is chipper, upbeat, active, sports-minded and energetic. However, unlike a redhead, this character is more feminine and less prone to blindly rushing ahead – and usually displays normal or above-average levels of intelligence. Note: darker shades of green typically carry similar connotations as “blue”.
As Kaneki had to repeatedly heal his toes over and over and over, all the while being tortured and borderline starved. His body was basically getting weaker and weaker because the cells were wearing thin, which is why his hair turns white, as happens with people when they get old.
While there have been a handful of anime that draw pubic hair on women (and an even smaller number that do so on men), anime characters are, by and large, body hair-free. The original reasons for this, of course, have to do with Japanese censorship laws.
Even traditionally Japanese anime characters can have hair of any color, even colors that don't traditionally appear on any real human! Like with manga, assigning different hair colors to different characters allows the viewer to recognize which character is which.
The natural hair color for Japanese people is generally black, of course. Long, black hair was a sign of beauty for women in the Heian period (794-1192), when Japan developed its own cultural preferences.