There is a common TV/Movie trope of shock or trauma causing a character's hair to turn grey/white almost instantly (if not overnight)
However, most "medical opinion" is that this is not possible since hair is not alive and so should not be affected by any biochemical changes in the body.
Nevertheless, there have been examples, although most are apocryphal and not backed by medically/scientifically acceptable evidence of drastic and rapid color changes to hair.
The "condition" is known as Canites Subita also known as Maria Antoinette Syndrome and a literature review reports as follows:
We have reviewed the medical literature on unusually rapid Canities of body hair to assess whether the reported clinical evidence can be explained with the current hypotheses of pathogenetic mechanisms. We screened the medical literature from 1800 onward, searching for as many case reports as possible. We assessed literature in Dutch, English, French, German, Italian and Spanish and included all cases, which contained an explicit mention or description of unusually rapid graying or whitening of hair. Case reports were classified into three categories: Cases are “authenticated” when the authors personally observed the rapid color change, “non-authenticated” when they saw the subject only after the alleged color change and “anecdotal” when authors were told about the case by a third party. In total, we found 196 cases of which 44 were authenticated.
These studies reported the graying of human hair in the context of aging, somatic diseases, emotional trauma or stress and psychiatric disorders. Numerous cases involved not only scalp hair, but also beards, eyelashes and other body hair. Several authors stressed that there was no alopecia.
Although plausible explanations exist to explain Canities subita occurring together with an effluvium, the observation of viable hair losing color along the axis within a timespan shorter than its growth rate remain as yet unexplained.