You seem to be a bit confused about grey / white hair. When people say they've got grey hair, they've actually developed a few purely white hairs - the hair loses its pigment completely. Because it's mixed in with normal coloured hair, it looks grey. Also, grey hair is thicker. Just telling you so you don't think you're some kind of freak.
Anyway, if it's just a few hairs and you're feeling unusually stressed, it will probably sort itself out. I found a few white hair when I was about 20, but pulled them out and haven't noticed any since. On the other hand, it is possible to go grey at your age. If you're a girl, dye it. If you're a boy, grey hair on a man is actually quite sexy - see Phillip Schofield.