year heavenly stem and year earthly branch

At which date do we base the computation for the heavenly stem and earthly branch, is it on our gregorian birthdate or do we use our chinese lunar birthdate.

I am planning to buy a watch cant decide what color (white, blue or pink) would bring me luck/prosperity/wealth? born gregorian: jan 20, 1991 (6:10am), female. hope you can help me. Thank you