Mapei makes one called SLATE that's a darker grey blue which I think would be better than WATERFALL. I hope it's still available. My samples are a couple of years old. TEC makes one called 973 WARM TAUPE which would be a nice neutral that would create great contrast. When your subway tile has a definite grout colour, the grid shows up very nicely. Hope I'm not too late with my assistance.

Cynthia, thanks for the reply. Unfortunately, SLATE also appears to be discontinued. I've been staring at photos of subway tile all day :-) . Our contractor has suggested white saying it would look "amazing". I had been anti-white grout (mainly due to worries about cleaning) but the more I've read today about darker grout is making me worried about that! Mainly that it will highlight any imperfections in the tiling. I'm also wondering how it would look given that the edges of the tile are rough not smooth. Will a darker grout look like the tiles are mistakes?