It's interesting how simultaneously universal and not this design is. On one hand, the clean angles and colors are obviously Metro; on the other hand, the station symbols seem to vary a lot from place to place. To me, the thick-lined circles of D.C.'s metro map are much more iconic, though obviously this is subjective and probably relative to which metro one is most familiar with!
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(http://www.washingtonpost.com/wp-srv/metro/images/metromap_021605.gif)
Hey, thanks for your input. The station symbol is designed after what you can see on the Cologne Metro map, which is my main reference as I have traveled it quite a bit over the years. It's used there in places where multiple tracks share a station
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(https://slack-imgs.com/?c=1&url=http%3A%2F%2Fontheworldmap.com%2Fgermany%2Fcity%2Fcologne%2Fcologne-metro-map.jpg)
I definitely want to stay away from text on this set, I do however have an idea for a small novelty kit to add some flare to the blank spots, will hopefully have that drawn out over the next weekend (hint: check the image above and I'm sure you can figure out what I'm thinking here). Tinted blank keys though, I'm not sure, maybe gray if anything or a slight blue (BFC color code) to mimic a body of water. But I feel there is not enough space to make those shapes justice.
This iconic style of subway/metro map design was originated by Massimo Vignelli in the 70's. Anything else is just an imitation. Vignelli had a very clear understanding of minimalist design. Many people mistake design as art—it is not. Design is a solution, that can (and usually does by nature) have artistic appeal. The variation of station symbols comes from less experienced designers taking arguably the best solution for metro maps, and trying to adapt it to their own version. I highly recommend revisiting the
"station symbols" in your design, as they would never hold up in the professional world of design.
As far as the "tinted blank keys" as you call them, or Modifiers as I was calling them—I think it would be best to go with an off white/grey color. This was actually one of the controversial parts of the original Vignelli design. When asked why he chose a neutral color for the bodies of water and parks he said, "Of course, I know the park is green and not gray. Who cares? You want to go from Point A to Point B. The only thing you are interested in is the spaghetti." For the same reasons he made this design choice, I think it would serve you well in your design. He didn't want you to pay too much attention to the bodies of water because they are very low on the hierarchy of importance. Their purpose is to give you a subtle reference to the geological features of the NYC area. For your keyset design, they would give the user a subtle reference to the "geological features" of their keyboard.