Plain sapphire is clear and colorless like glass. 

If you add about 1/20% of TiO2 to Aluminum Oxide (Al2O3) it makes sapphire light pink.

If you add about 1/20% of Rust (Fe2O3) to Aluminum Oxide it makes sapphire yellow.

If you add both of the above, it turns sapphire blue...which is the color for September.

If you add about 1/20% of Chrome Oxide (Cr2O3) it also makes sapphire pink.

If you add a little more Cr2O3 it make sapphire red which is then called Ruby.