Can Fear or Stress Change Your Hair Color?
Any extraordinary emotion can change the color of your hair, but not instantly. Your psychological state has a significant impact on the hormones that can affect the amount of melanin deposited in each strand of hair, but the effect of emotion takes a long time to see. The hair you see on your head emerged from its follicle a long time ago. So, graying or any other color change is a gradual process, occurring over the course of several months or years.
https://www.thoughtco.com/can-hair-turn-white-overnight-604317
From that SAME ARTICLE:
Medical Conditions That May Explain Hair Bleaching
Your emotions can’t instantly change the color of your hair, but it is possible you could turn gray overnight. How? A medical condition termed “diffuse alopecia areata” can result in sudden hair loss. The biochemistry of alopecia isn’t well understood, but in people who have a mix of dark and gray or white hair, the uncolored hair is less likely to fall out. The result? A person can appear to go gray overnight.
Another medical condition called canities subita is closely related to alopecia but may not involve the loss of as much hair. According to American biologist Michael Nahm and colleagues, “Today, the syndrome is interpreted as an acute episode of diffuse alopecia areata in which the very sudden ‘overnight’ graying is caused by the preferential loss of pigmented hair in this supposedly immune-mediated disorder. This observation has led some experts to hypothesize that the autoimmune target in alopecia areata may be related to the melanin pigment system.”