Exogenous chromonychia is a rare but important diagnostic consideration in cases of nail pigmentation. We present a 69-year-old male with progressive, asymptomatic grey discoloration affecting all fingernails for over 2 years. Extensive workup, including histopathology and fungal studies, was negative for infectious or melanocytic pathology. Detailed history revealed long-term use of a grey-reducing hair coloring shampoo, containing 1,2,4-Trihydroxybenzene (1,2,4-THB), a compound linked to pigmentary changes. Following discontinuation of the shampoo, the nail discoloration nearly resolved. This case underscores the significance of thorough exposure history in evaluating melanonychia and highlights an unusual but ubiquitous exogenous cause of nail pigmentation.