Question

Why do we have black hair

Answer 1

pigments give hair its color, namely eumelanin and pheomelanin.
pheomelanin:people who have this will have orange and red hair
Eumelanin:people who have this will have either black or brown hair

Answer 2

The colour of our hair is decided by two pigments---Eumelanin and Pheomelanin. If a person has high quantity of Pheomelanin then the color of his hair will be red or orange but if he have the quantity of Eumelanin more then the color his hair will be black or brown as eumelanin determines the darkness of the color of hair.