Yes, you can use different colored grout for the floor, but as I said in the other thread, the Platinum would work well for the hex, as well. The only other color I'd personally consider would be the Delorean Grey. Which ever one you use, though, you want to use sanded grout on that hex tile.

Of the four colors you mentioned (Mapei Silver, Custom Platinum, Mapei Warm Grey, and Laticrete Silver Shadow) The only one that's darker than the others is the Mapei Silver. All the rest are about the same tone. As for other manufacturers, you could also look into TEC and Hydronment. I don't have the TEC colors available, but as for Hydoment, look into Silver Bullet or Mist.

Bumping for another try. I also posted on flooring--hopefully someone knows.

Thanks, all. Prickly and linelle: Very perspicacious of you! I should have explained this. The tiles ARE indeed a bit different. I used the tiles that I liked least in the locations that will go under cabinets. I also did my test-grouting on areas that go under the cabs. Ergo, the right-hand (grouted) side has less vibrant tiles, with lighter reds and darker tans. This effect is more dramatic in other areas of the kitchen; I chose the "least bad" area to show. I believe I got all the grout haze off. Thanks, annsch and breezy. I am so bad at these things that it is helpful to have them discussed! The concept of the red-and-white checkerboard was meant as a subtle nod towards vintage kitchens in general, with Marmoleum/Linoleum floors in mind in particular. One realization I had since asking this question on this thread was that a grout that blends in likely comes closer to this look (since Marmoleum didn't have any grout lines to delineate the checkerboard). If you look at the picture above, you could imagine the right-hand side was marmoleum, but you couldn't imagine the left-hand one was.