Transition metal oxide nanoparticles have been extensively studied for the development of smart windows which are expected to be a promising technology to save energy in buildings. However, most of them turn blue under UV irradiation. Since the blue coloration affects the color of objects through the windows, the development of materials with a neutral color which hardly disturbs the view is more beneficial. In this work, we prepared a colorless-transparent TiO2 colloidal solution containing Mo6+ ions (Mo-TiO2), which turns black via gray in a nitrogen atmosphere under UV irradiation. An absorption peak was observed at 535 nm, which increased with an increase in the UV irradiation time and reached a constant value (Absmax). The Absmax value increased linearly with an increase in the Mo doping amount. We demonstrated that the photochromic behavior of the Mo-TiO2 nanoparticles is completely different from that of pure MoO3 or the mixture of MoO3 and TiO2. In addition, we performed a kinetic study to elucidate the mechanism and found that the coloration rate at the initial stage decreased with an increase in Mo doping amount. Based on the kinetic analysis, the following results are obtained: a color center is formed at a deeper energy level than the Mo dopant level; the number of the color center depends on the Mo doping amount, and the color center traps the photogenerated electrons more rapidly when it is isolated. The black color was bleached by purging the air in the solution. In particular, the gray state which is observed as a transient color is promising for the development of smart windows to shield the sunlight while allowing a clear and undistorted view.