Cecil described selective hair loss under stress, with relative sparing of white hairs, as an explanation of hair seeming to turn white overnight, or at least very quickly. I cannot absolutely confirm that, but I can relate some information that supports it.
I am an resident physician in a veteran’s hospital, and recently had a patient who was a veteran of WWII. He had served on board an ammunition supply ship, and he told me how shortly after the D-Day invasion, his ship anchored off the coast of France. They were under a great deal of stress, since the ship was so vulnerable to enemy attack, being loaded with explosives and other ammunition. They had to remain at their posts for several days nonstop, during which time, he told me, many of the men either went completely bald or their hair turned white.
If rapid whitening of hair is due to selective alopecia of non-white hairs, it seems reasonable to me that young men who had no white or gray hairs, might lose all their hair in the same situations in which others might appear to spontaneously turn gray. That appears to have occurred in this case, although it seems equally reasonable to implicate nutritional factors and possibly chemical effects related to the cargo on the ship. And finally, my information is only second-hand.