Scientists discover 'why stress turns hair white'
I can agree because I knew it. Almost half of my hairs turned grey 3 times. It always occurred after significant stress which was 1995, 2005, 2012. However, I reject the general assumption that the change in natural hair color is permanent. Most of my hairs turned brown again after a few years without stress.
So, if I grow mice hair on my head, and if I worry,,,,,,,,,,,,,I don't get it.
In my first marriage i started to get the usual friar bald spot and knew it was stress related. After we broke up the spot disappeared and now plenty of hair.
I'm reminded -- "anecdotes are not data" 
Wow! This runs counter to my experience. I have had extreme stress in my life, but the majority of my hair is still brown at the age of 66. I am gradually going gray in the same pattern as all the women in my family.
I'm reminded -- "anecdotes are not data"