A good tiler will advise on which grout will work best with your chosen tile in most cases. However, to make your own decision, there are generally three options; contrast, complement, or camouflage.
A contrasting grout colour such as charcoal grout paired with a white tile really makes the tiles pop.
A complementary grout colour, such as blue-tinted grout on a white tile with blue accents, will match the tiles colour palette.
A camouflaged grout colour, such as white grout paired with a white tile, blends the tiles and grout for a seamless look.