Grout selection depends on the look you are trying to achieve along with other factors. Lighter grout will darken slightly over time and shows dirt more. That can be kept to a minimum using a grout sealer. Dark grout hides dirt and water stains. When choosing your grout color, you can either match the tile for a blended look, or you can choose a contrasting color which will make each tile look bordered. Epoxy grout is the most resilient but is difficult to remove should you need to replace a tile in the future.
Michael Prachniak - Expert Tile and Stone Consultant and Writer, Owner - Tile Productions
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