The floor has a marble hex tile with silver grout (a medium neutral gray) and it looks fantastic. Should I just go with white grout or do a light gray like custom's platinum color?
It's probably a matter of taste, but if you are going to a grout color that does not match the tile, you'd better have a perfect tile job because every flaw will show.
I prefer the look of matching grout as it makes it far less busy....
Do you want the shape of the tile and the layout of the tile to stand out? If so, this would be more visible with contrasting grout. If you want more of a clean white palette with minimal attention drawn to the tile shape and layout, use a grout that matches the tile.
I would say white. That way your accent tile (gorgeous) in the niche will stand out.
That said, that's what I did in our one shower.
i ended up doing white grout. it's really pretty, but i wish i would have done a very light gray. i'll do light gray on the other shower's subway tile i think. thanks for the advice, everyone. i'll take photos tomorrow. oh, and my tile guy replaced the broken tiles with no problem.