TLDR  Vitamin B12 deficiency can cause skin and hair color changes, which are reversible with treatment.   
  A case study from 1986 reported reversible generalized hyperpigmentation of the skin and nails, along with premature gray hair, due to vitamin B12 deficiency caused by pernicious anemia. The patient's skin and hair pigmentation returned to normal following treatment with intramuscular cyanocobalamin.
          
                 11 citations
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  August 1986   in “Archives of Dermatology”    Vitamin B12 deficiency can cause skin and hair color changes, which can be reversed with treatment.  
     
           50 citations
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  November 2010   in “Otolaryngologic Clinics of North America”    Recognizing oral symptoms can help diagnose and treat blood and nutritional diseases early.  
     
           January 2011   in “Springer eBooks”    Eating a balanced diet with the right vitamins and minerals is important for healthy hair, but too many supplements can be harmful.  
     
           October 2022   in “Southeast Asian journal of health professionals”    Hair greying is seen as a sign of aging; temporary fixes like hair dye are used, but a balanced diet and hair care can help manage it.  
     
           August 2024   in “Journal of the Association of Physicians of India”    Adding vitamin B12 to food like salt can effectively improve vitamin levels in rural areas.