Once your hair loses its colour, the colour is gone for those follicles. You have to dye it if you don't want white or grey hair. The only way for white or grey hair to stop growing is for hair in general to stop growing- i.e., you go bald. I started getting my first white hair when I was 24 as dad's side of the family goes grey early. However, I didn't start dying my hair until I was 30 and there was enough of it to be noticeable. I might eventually stop dying my hair and let my white hair show, but not yet as I'm only 36. Maybe when I'm 45 or 50, but at 36 I feel odd having lots of white hair, so I dye it.