There is a few hair colors that are pretty rare. I happen to be a redhead and people usually say it's unique. Only 1-2% people have it.
Yes its really rare, i thing that its going extinct :D i happen to have brown hair, pretty standard :D
From scientific point of view hair color is dependent on the presence of eumelanin, the more there is the darker the hair is. Much like a skin color depends on melanin.