There's a story that Marie Antoinette, Queen of the King of France Louis XVI, had her hair go completely gray overnight because of the stress when she figured she will be executed soon. However, considering how hair is made, this story seems unrealistic. The hair’s color would never change dramatically overnight unless you apply color to your hair.
Hair color does not change after growing. The hair color is dependant on the type and amount of melanin pigment. The melanin pigment is created from the roots along with the hair as it grows. The hair is pushed up from the roots to the scalp and will come out of the scalp as a natural color such as blonde or black hair. The color of the hair hardly changes afterwards.
So, what kind of mechanism will cause gray hair to come out? Gray hair grows out from the scalp without pigment when the roots cannot produce melanin pigment for some reason (genetic, aging, or stress).
In other words, the gray hair is white from the beginning before the hair growth, it is rare that the colored hair will naturally become gray hair all of sudden.
Some causes of gray hair are genetic, aging or stress. So then why was there a story that "Marie Antoinette's hair turned gray overnight"? The reason why melanin pigment stops production in hair root has not been fully understood yet, but stress is considered to be a major cause as well as heredity and aging. According to the record, "Marie Antoinette was executed about two months after being transferred to the prison", and it seems that her hair was short at the time of execution. Moving from a luxurious court life to prison is perceived as unimaginable stress for Mary Antoinette. Perhaps her gray hair increased suddenly due to the stress of prison life, and it seemed that it suddenly became gray hair after having her hair cut short before the execution.
Maybe it looks like gray?
Also, Marie Antoinette may not have been gray hair on the day it was executed. When she was living an enjoyable life at the royal court she was also seen as a fashion leader. It is also thought that the "dull blond hair" which was not clean or cared for on the day of execution was reflected as gray hair in the eyes of people as the impression of their well-maintained hair and dressed up Queen was strong.
Although it is not comparable with Marie Antoinette’s situation before execution, we all live with various modern day stress. For the benefit of our hair as well as for the body, we should to work to eliminate stress and try to live as healthy as possible.