I dunno, basically everyone here is on the gray grout kick. I did my bath in subway tile, and planned on silver shadow grout. my tile guy talked me out of it for white. He was somewhat insistant (we also knew him, so we weren't really strong armed) I was going to stick to my guns, but with the mosaic I wasn't really sure, and after seeing it ungrouted and pictured with dark, I was basically on the fence on the gray thing. So I went white. looking at the finished product, I'm glad I listened, it looks great in my bath. If it looks darker in person than in the pics I'd change it out (assuming its not a huge deal to do) whether to go white or silver, it depends, I can see both as a possible. but lets be realistic, if you "live with it" you're never goign to get the tile guys back to change it out unless you absolutely hate it and want to change the whole thing down the line.

I have the Subway Ceramics Avalon / Minas with a medium gray grout. And a Shaw's white sink, if anyone is considering that, by the way. (My contractor could have ordered it directly, but at his request, I went through a tile store, and by the time everyone had put their sugar onto it, it was OBSCENELY expensive.) I'm not an installation expert, but at least with the Subway Ceramics, the installers use little plastic spacers between the tiles (I think some others have their own spacers?) when they put them up, they don't do it freehand, so how much grout you get is a function of 1) the limits set by the curvature of the tile and 2) what size spacers they use. So you might be willing to go a little darker with a smaller grout line, or with a handmade tile that refuses to go close together, you might want to stay light. The traditional color is close to concrete. But most modern people don't seem to like that, so do what pleases you!

kegrant- I too would love to see photos and do post on what you finally decide. I too have simply white cabinets (ikea lidingo) and soapstone counters and am leaning towards white subways with white or possibly silvery gray grout (DH's preference) but everyone who looks at my kitchen says you want to add more white?!?! I am pretty confident I will love the look, but photos are always better. Best of luck to you in your search!

How much contrast do you want? If you've got light colored subway tiles & select dark grout you'll emphasize tile shape & if your tiles are handmade (not all perfectly same size) remember dark grout may highlight uneven grout lines. (Some people like that, others are upset...all about your expectations!)