Mercury is not used anymore out of price and toxicity reasons. Current mirrors use silver and less current, aluminium.
Ti02 is not white. In fact it is transparent. But all powders look white because they reflect the white light in all directions. This is the case for sugar and salt. Both are transparent and look white. I do not know for sure why Ti02 is better than other bases used for paints, as ZnS. I presume that it is due to its high refraction index (2.6-2.9) that makes that more light is reflected each time it enters a crystal.
I you put Ti2 powder in the back face of a glass, you will get a very white glass, but not a mirror. If you are rich and put a monocrystal, you will get something like a glass but for which the reflected image is stronger than that of glass.