Why does hair turn grey?

Hair gets its colour from melanin, made by melanocyte cells in the follicle. With age those cells produce less and eventually stop. Hair that grows in without pigment is actually colourless; it looks white or silver because of how it reflects light.

One thing follows from this: a hair that has already grown cannot change colour. Hair is made of dead cells, so the change always happens at the root and shows only in new growth. That is why “his hair turned white overnight” cannot be literally true.

When it happens is largely genetic — if your parents greyed early, you probably will too. Research suggests sustained stress can accelerate the process, but stress alone is not what causes it.

The belief that pulling out a white hair makes several more appear has no basis either. Each follicle grows one hair, and removing it does nothing to its neighbours. Repeated pulling can damage a follicle, though, so it is not a habit worth keeping.