Your natural hair colour is dictated by the amount of melanin present in your hair; the more melanin the darker the hair. There are two types:

Eumelanin:  a black-brown pigment determining intensity
Phaeomelanin: which creates blondes and redheads.

Why do we go grey?
Grey hair is created when the melanin production decreases in the root. When you go grey is also dictated by your genes so it’s not strictly a sign of ageing.

Why does dark hair appear shinier than light hair?
The shine of your hair depends on the density of melanin pigments in the cortex as well as the condition of the colourless cuticle that coats each shaft of hair. If the cuticle is rough it catches and absorbs the light producing dull hair, if it is smooth it reflects the light giving of a healthy glow and accentuating the colour in the cortex.