What turns corundum blue?

Question

and what color would titanium turn it? (if not blue)

Answer 1

You might want to read the Wikipedia article on Sapphire; http://en.wikipedia.org/wiki/Sapphire

It says:

"Various shades of blue [dark and light] result from titanium and iron substitutions in the aluminum oxide crystal lattice."

Note that it says titanium "and" iron, not "or" I don't know what color  titanium alone would give