response: It has to do with pigment. So high levels of eumelanin makes the hair dark while lower levels of pheomelanin make the hair lighter.
history: I have brown hair, quite common I guess. Yes, it is below black hair for most common hair color. And what is the least common hair color? Red hair naturally happens in 1 to 2% of the human population although I'm not sure fi it is the least common hair color. I say 1 or 2% probably makes it the least likely. Do you know what makes hair dark like my brown?
knowledge: It is characterized by higher levels of the dark pigment eumelanin and lower levels of the pale pigment pheomelanin.