The color of grout selected for your project will impact the finished look.
For a uniform look, select a grout color that is a close match to the tile for a tone-on-tone subtle effect. This will make the room look bigger. Choosing a shade that is just slightly different that the tile color will give your installation more depth and interest.
Consider where the grout is going to be used; a darker shade tends to show less dirt.
To emphasize each tile or a tile pattern, choose a grout in a contrasting color. The grout then acts as a frame for the tile, giving the grid effect.