Hossman is right. In October 2008 I went through something extremely physically and emotionally traumatizing. Just today (which is why i googled this topic and found this thread) i was looking closely in the mirror, and noticed a bunch of only half white hairs. I have almost black hair, and don’t color it, so it’s very obvious. I had noticed the white hairs since October, but never noticed the clear and NOT gradual line where my black hair turned white. It’s bizzare, the top 3 inches closest to my head are stark white, and the rest of the hair is black. It’s only about 6 or 7 hairs, but it shows a clear point in time where the follical completely stopped producing melanin “overnight”. So, No the whole hair can’t turn white, but during a trauma, the cells can immediatly stop producing the color, so the REST of that hair will be white. My hair is proof.