I would go with gray grout - white is difficult to maintain. I have white grout now and would never use it again. If you are iffy on the gray, just go with a lighter gray color...just dark enough so it doesn't look dingy. Good luck!

We used Bostick quartzlock2 silver grey on some of tile. It kind of has a bit of a warm tinge to it. Its not as silver grey as I thought it would be but slightly a grey with a brown tinge to it. It might work well for you. The urethane grout should not stain like regular grout. I do know its a lot harder to work with though.

I know that yogurt commercial picture is tough to really see the grout. But any suggestions for a good light tan or simply off white grout color to use?

not contributing anything new-darker, or rather contrasting, grout when you want to emphasize the shape or the pattern of the tile, blending-close to the color of the given tile grout-if you don't here's another thought for you..light gray/bone will be probably fine. the darker you take it-the stronger the contrast. which might be amazing looking and all but take into consideration-not everybody is doing great with high contrast, especially very dark and very light together, colors and lines interchanging frequently as in with the grout and tiles..it makes a lot of people dizzy-not good for a space like a bathroom where you'd rather not feel dizzy if it was for your own enjoyment and you handle high contrast well-go for it if it's for resale-take into consideration a pretty big amount of people that will find it hard on their eyes i also wanted to go with contrasting grout..fortunately i realized i'd be the first to fall in my own shower, surrounded by this striking geometry again, if it's just a tad darker color though-it will probably look cool and still be fine with the majority of the buyers