We report a novel photoreversible color switching system based on the photocatalytic activity of TiO2 nanocrystals and the redox-driven color switching property of methylene blue (MB). This system rapidly changes from blue to colorless under UV irradiation and recovers its original blue color under visible light irradiation. We have identified four major competing reactions that contribute to the photoreversible switching, among which two are dominant: the decoloration process is mainly driven by the reduction of MB to leuco MB by photogenerated electrons from TiO2 nanocrystals under UV irradiation, and the recoloration process operates by the TiO2-induced self-catalyzed oxidation of LMB under visible irradiation. Compared with the conventional color switching systems based on photoisomerization of chromophores, our system has not only low toxicity but also significantly improved switching rate and cycling performance. It is envisioned that this photoreversible system may promise unique opportunities for many light-driven actuating or color switching applications.