Take a tile and go look at grout colors you want one that is as close as possible to the tile

A high performance modem grout is a lot more expensive than the old sanded or I sanded grouts. For a reason. Technology. Choosing a cheaper product, and the wrong product was an error. They should have discussed this with you, given you the change order upcharge, and used the top quality product. You wouldn’t be having this issue. Installers used to old fashioned grouts don’t like it, because it’s more work and requires a helper to be sure it’s cleaned in a timely manner. So it’s an upcharge on labor as well as materials. But it’s a once and done. https://www.lowes.com/pd/MAPEI-Flexcolor-CQ-1-Gallon-Cocoa-Acrylic-Premix-Grout/50360276 You can still go over what is there with a colorant. But only provided that it’s steam cleaned of any of their “cleaning” residue, , allowed to dry, and there is no grout loosening or coming out of the joints. You have to wipe it with a clean white rag and make sure there is no residue coming off after cleaning. That would indicate an improper mix with too much water in it. https://www.lowes.com/pd/MAPEI-Grout-Refresh-8-fl-oz-Cocoa-Ceramic-Porcelain-Tile-Grout-Sealer-Squeeze-Bottle/999977878

Neither. I like a lighter one, but not the one you picked. that one is too red. And for sure I would NOT go dark. Mapei, left, two on the right are Custom Products. I like these colors better. this is their Dapple Gray Merona tile. your lines will be like this. but notice how the grout color is an exact match. neither too dark or too light. light grout. more rustic looking tile, same grout spacing, darker color. I think w/this flooring you could get away w/a darker grout color because of the darker veining colors. this one used a darker grout, but the grout lines are thinner. I wouldn't like it if it had the thicker joints Merona Natural tile.

Amy10N: I suggest if you want a darker grout color, you need to take a few of your tiles to a tile store and pick out a grout color that will blend well with your tile. Each grout manufacturer will have different names for their grout colors. When I purchased my Italian porcelain wood look tiles, I also purchased a grout color that blended well with the tile.