Myth: Hair can turn white overnight.
May 23rd 2017 by Tony Maleedy.
Not true. Hair can turn white quickly … but not that quickly. Hair colour pigment (melanin) can stop being produced within a short time because of such things as stress or ill health. If this happens to many thousands of hairs at the same time a person’s hair can turn from being coloured to grey or white within a few weeks or months.
The science bit:
Once hair has grown out of the hair follicle the only things that will affect its colour are hair colouring products or environmental effects such as ultra-violet radiation from the sun bleaching the hair. Melanin within the cortex of the hair cannot simply disappear without chemical intervention.