Grey Hair!?

I do know that when your hair turns grey it's because of loss of pigment at the hair root. BUT how come you never see 1/2 grey and 1/2 colored. I'm obviously not talking about those who dye their hair. Your hair is either brown (let's just pretend) or grey. You never see hair that's brown then starts to turn grey at the root. Does anyone know why?Grey Hair!?
actually i have seen this in my own hair. i found a couple of strands that were just starting to turn gray. it was gray from the root to about 1/3 of the strand and the rest was my natural hair color.