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!

I'm using a matte subway in an off-white, but I selected a grout as close to the tile color as possible. It was all about what I was trying to accomplish for the tile. I would not change grout based on gloss/matte. I would go by the color that gave me the overall look I wanted with the tile. The subways I'm using are handmade, so they have wavery, imperfect edges, and they're probably close to 1/2" thick. I want to enhance the shape/texture of the tile itself, so I opted for a grout that would disappear. If you want to enhance the presence of the tile a bit more, since yours has a lower profile, then I think a light grey would look nice, especially with the countertops. Have you looked into the beveled subways? They might give a nice enhanced tile look, too, similar to the use of darker grout.

If you have tight grout lines, you would be using unsanded grout, which looks whiter than the sanded version of the same color. We recently used TEX XT grout in Bright White--#910 in our bathrooms, and it looks very white. The large format Porcelanosa tile we used in our MB is a very white, very glossy tile, so we wanted a white grout that matched well. The XT is supposed to be more stain resistant (than other grouts?), so hopefully our white grout will be less likely to discolor (since it looks so pretty right now!). It's not necessary to seal it, but you can (with a solvent-based sealer). We sealed ours for added protection/peace-of-mind.

I'd do a white grout, to match the wall tile. You already have a lot of pattern going on with the hex and the ceramic tile. Do you feel you want the subways to pop dramatically like they will with black grout? I don't think I'd do grey, afraid it would look off in contrast with the black.