Ok so you've decided on white tile. They will still ask you a few more questions....one of which will be grout color.
Darker grout will hide dirt in a floor, is a bit more trendy, and provides contrast. If you have an interesting shape you want to highlight, a contrasting grout will help.
Lighter grout gives a softer more calm feeling.
Which do you like best? Would you use one look in a kitchen and another in a bathroom? Do you want your tile to be the star or more of a subtle background to highlight another feature?