Jaabir bin Samurah RAwas asked a question regarding the white hair of Rasulullah SAW.
"When Rasulullah SAW oiled his hair, it (white hair) did not show. When he did not oil it, it showed."
Here, the Scholars of Hadith have explained that oil gives the hair a shine. Therefore the white hair becomes indistinguishable from the other hair; or they become compact and mixed. This therefore explains the above narration whereby the few white hairs of Rasulullah SAW cannot be seen easily. In contrast, when there is no oil, the hair becomes dry and loose, and thus - white hair and black hair become easily distinguishable.