It's usually best for the grout color to match the tile color as closely as possible, especially for a traditional look. If you are going for a very contemporary look however, and you like a high-contrast, active aesthetic, then a darker grout color would be okay. Hope that helps!
Be VERY careful with a darker grout. Tile installation must be flawlessly level and perpendicular. Dark grout shows every flaw no matter how tiny it is.
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