The difference in color of the TiO2 is due to the degree of oxidation. Fully oxidized TiO2 is white. With even a slight reduction, the material becomes grey to black in color. When evaporating TiO2 it is necessary to melt the material before depositing films. Titanium has several stable oxides (TiO, TiO2, Ti2O3, Ti3O5) and when the material is heated it evolves oxygen and turns black. Regardless of which material you use to start (white or black) the material will turn black and evolve oxygen when it is heated up. Once outgassing subsides the films can then be deposited. It is necessary to add 1 x 10^-4 Torr to 2 x 10^-4 Torr of O2 gas during the deposition to maintain stoichiometry of the film.