I just came across this thread, and don't know if anyone is still reading, but wanted to write, because I just put up my backsplash (in a new white shaker style kitchen). Everyone who sold the white subways said to use white for the grout. But most people in Gardenweb were saying to use gray? I didn't end up buying at home depot, but a guy there said it best. He said that the "authentic" color of grout used with subway tiles at the beginning of the century was always white. I told him mine looked creamy to me, and he said "If the tile reads white, at all, when you look at them, use "bright white". When you see subway tiles in a movie and the grout looks gray, that is done to make them look very old and that the grout has gotten dirty/discolored after many years. He said it is was in the 1980's that gray or black grout started to be used with a white tile. The brick pattern doesn't dissapear with white grout. You can still see the brick pattern under lighting because the tiles aren't totally flat. It's a cleaner look with white and authentic to the early century rather than the 80's. The original subway tile did not have beveled edges. This allowed a smooth surface as opposed to the modern tile which really defines separation in the installation.

We have white grout (with white ceramic tile) and have had no problem with staining. I have frequently splashed bits of tomato and orange onto the backsplash (think canning and juicing) and had no problems wiping it off. Even when I find errant splots two days later, it just wipes off.

Many glass manufacturers do not allow the use of epoxy grout with their product. You must contact the manufacturer for this information. If there are no instructions in the box also inquire as to which setting material is called for. Some glass may require mastic instead of mortar. While you have them on the line ask about thermal shock rating. You have a large format glass tile (9 square inches is large format for glass) If you have a cook top, especially gas simply boiling a pot of water on the back burner could crack the glass tiles due to thermal expansion. Just a few things the DIY shows and box stores won't mention. Was any of this mentioned to you at your point of purchase? Always test grout first to verify it will not scratch the glass. Other grout manufacturers such as Ardex, Bostik, Tec should be considered also. Some make grout specifically for glass. I do not recommend glass tile installation for a DIY'er. or a GC/ remodeler. There is a lot more to it that I did not mention. Proper cutting, setting, polishing etc. If you think hiring a professional is expensive wait until you see the cost of hiring a amateur. Good luck with your project.

I'm guessing you have additional tile left over. You should anyway for future repairs if needed. My suggestion is to make up 3 "panels" of the tile on say a piece of plywood. Whatever thin set color was used to apply the tile to the spa should be the same. With the tiles being a bit transparent, the thin set color will show through. Grout each with the different colors, and allow them to fully dry. Place all 3 next to the spa in the daylight, and exam the color differences. This should help in your decision making. The coping will cast a slight shadow at the top of the tile so have someone hold the tile panels against the spa, under the coping, to duplicate the conditions. Consider your plaster finish coordinating when making this decision also.