I think Byz's link answers the question. I think there's a misunderstanding here - yes stress/trauma can cause a person to prematurely age and turn gray faster. The hair on your head is dead and can't change color unless you chemically alter it or bake in the sun. So if a person goes through a sudden shock the hair already on their head won't turn gray/white, but the new growth might start growing gray. Genetics and stress does play a role in prematurely turning gray, but a stressful event won't do anything to the hair already on your head.