Hair greying (referred to during ageing as ‘canities’) is one of the earliest and most visible indicators of ageing in humans. The social significance of greying persists across cultures, geographical locations, and ethnicities, alongside a now-widespread interest in its reversal.
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There is still no universally accepted model of human hair greying, and the extent of genetic contributions to greying remains unclear.

Source : The Biology of Human Hair Greying Biological Reviews, Volume 96, Issue 1 p. 107-128.

In addition to the uncertainties about the processes behind long-term hair greying, there are also numerous substantiated reports of sudden greying (Canities subita) apparently triggered by psychological and/or physical stress.

A 2013 research project reviewed nearly 200 reported cases, concluding :

[…] the observation of viable hair losing color along the axis within a timespan shorter than its growth rate remain as yet unexplained.

See : Canities subita: A reappraisal of evidence based on 196 case reports published in the medical literature International Journal of Trichology 5(2):63-68

Update Apr. 2023. A research team from New York University suggest that the greying may be a result of malfinctions in 'melanocyte stem cells', or McSCs (in mice).

The organization of the McSC system, responsible for hair pigmentation, is thought to parallel that of hair follicle stem cells (HFSCs)"

See: Nature, 2023.

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