A person's hair can change a bit depending on sunlight exposure (it "tans"). Japanese black hair often turns reddish. A blonde's hair can vary quite a bit.
The same hair might be described as "chestnut brown, flaxen, blonde, wheat, etc" by different people depending on their color skills
Bleh, I suppose this is off-topic. I'll move it if the same questions appear in the Holo character thread.