Jaabir bin Samurah R.A. was asked a question regarding the white hair of Rasulullah (Sallallahu alaihe wasallam). He replied: "When Rasulullah (Sallallahu alaihe wasallam) oiled his hair, it (white hair) did not show. When he did not oil it, it showed".
Oil gives the hair a shine, therefore the white hair becomes indistinguishable from the other hair, or they become compact and mixed, therefore the few whit hairs cannot be easily seen. When there is no oil the hair becomes dry and loose, and is easily seen.