It's not the same process as turning grey. Hair colour is a complex system controlled by more than on gene. Actually it is a set of genes, and the ones that are active depend on your age. As you get older, the horomone balance in your body shifts, and your "adult" genes take over. So light blond hair can become dark blonde, or brown, or red hair may turn auburn. In a few populations blonde hair stays blonde. For most, it darkens with age. Presumably long ago there were evolutionary survival reasons for this change in hair colour with age, but that is just speculation. Hair turns grey when the body stops making the pigment that colours hair. This can happen for various reasons, but with aging the pigment-making system just gets old and wears out.