If you want to create contrast between white marble tile and grout, consider using a darker color for the grout. This will create a striking contrast that can help highlight the unique beauty of the marble. A popular choice is gray grout, which can create a modern, industrial look.

If you want to create a pop but in a less drastic way than you would get with black grout, a light gray grout or medium gray grout is the way to go. It will also add a nice contrast against the white subway tile, but won't be as punch-worthy.

If you're trying to unify your tile, such as with a wood look tile floor, go for a close match that's a little bit lighter than your tile. If you want to highlight a color or accent in your tile, like the gray veins in a marble tile, go for a close match, or a bit darker than that shade in your tile.

For balancing aesthetics and upkeep, you can't go wrong with a neutral grey or beige grout. These lightly pigmented grouts will enhance the pattern of both light and dark colored tiles, have less risk of staining your tile compared to a dark grout and will conceal dirt and stains better than a white grout.