There are many ways to coloring on the metal such as plating, painting etc.
Coloring of titanium is mainly by making thin film on the surface by special method.
This thin film is strong against corrosion, so, keeps the material long period without discoloring or color changing

This film is also strong against acid rain or sea wind. Out door titanium colored monument and art is good for titanium.

Titanium color is controlled by the thickness of the surface film.

Titanium is beautifully brilliant as jewelry. 
　
What is the color of titanium?
It is difficult to say the color of titanium.
May be, we can say titanium color is rainbow color