I have brown hair(s) and gray hair(s) but no hair that is half down the shaft is brown and other half is gray.
Does it realy lose all its color at once?
If it {follicle} grows a brown hair then, when that hair falls out why it Now grows gray hair(s)? You know it had enough pigment to finish the last hair.
Silly question but can get it,
Good cite, raisinbread, but it doesn’t answer the OP question of: Why do the melanocytes always stop producing pigment after one hair falls out but before another grows in its place, instead of in mid-hair? Or is the question based on a misperception?
A hair follicle produces continuously for a long time. On a healthy scalp for several years.
Then it goes into a resting stage. Growth stops, cell crumples down a bit and hair falls out.
So losing a hair is a completely natural process, somewhat similar to falling leafs in autumn, unless you tear on it.
The rest again will last some time.
( If you have enough active neighbours you will not get bald in the meantime. )
Then, hopefully, some day this one follicle gets busy again. Possibly in the meantime it aged enough to loose its ability to produce melanin. The new hair is grey.
So there is quite some time lap between coloured or not and no funny getting grey over night or within the stem.
MummyCave
I’m more curious about people who have skunkboy hair meaning a single white patch amid their luxuriant locks.
I’ve got a patch of white hair on the left side of the back of my head. Most of my hair is brown except the spot, which I’ve had my whole life. My mom also has a white area in her hair on the top of her head. It’s fun to tell gullible people that I got struck by lightning when I was three years old. They ask me what it felt like, and I just say it knocked me out and don’t really remember. 
So does anyone think those stories about people
being frightened their hair turns white suddenly?
Falling off a ship and being rescued , and other stories
I have read.
Cecil on hair turning white overnight: http://www.straightdope.com/classics/a2_315b.html
Cecil on grey hair: http://www.straightdope.com/classics/a1_267.html
There is a town in southern Alabama called Skipperville. Shortly after meeting my wife we were driving from Pensacola to Atlanta with a friend of mine. We passed a sign pointing to Skipperville and my friend and I remarked about it being a funny name. My wife (whose mother’s family is from those parts) said that it was because almost everyone in that town was named Skipper. She went on to say that you could tell if someone was from Skipperville because they all had a spot of white hair somewhere on their head. So now when I see someone like that I wonder where they are from.
I have a place just above my forehead on the right where the hair has grown in white since I was 10. I got hit in the head really hard by a curb.
I was the only kid in the 5th grade with grey hair! It went great with the glasses and braces. :rolleyes:
I remember several years ago, my great-grandmother let me comb out her hair. I guess I had assumed it was just short hair all this time because it had always to my memory been up in a sort of spiral flat to her head with pins. Anyway, she let it loose and it fell to almost her feet.
The hair to her mid-back was white, almost clear. From there it started to darken slowly until by her ankles it was light brown. It was just amazingly beautiful and I swore I’d never cut my hair again, just so when I’m 90 I can look like that.
So I can tell you firsthand that at least in one case, hair does change in mid-strand.