Okay, so final thoughts on this set. Keep in mind I've never owned SA Penumbra and have never seen it in real life.
The main reason I fell in love with SA Penumbra years ago was the navy blue modifiers mixed with the colored legends. The warm colors of everything--from the Solarized alphas to the yellowed ones--screamed mock-vintage, and I was willing to forego the fact that it was SA. But SA profile also lent to its invitation with the large caps and exaggeratedly sized legends.
So with GMK, I don't think comparing them is fair. This is Penumbra in name, but the results are different. When I first pulled them out of the slip-case I thought it looked great. Then later this afternoon I had the chance to pull them out completely and found myself a little underwhelmed. The modifiers were not as blue as I had hoped, and they appeared more black. But I wound up placing them on my keyboard (titanium gray Jane) anyway, and now I really like the set again. I have them paired with Hydro novelties, so I think this will look really good with Solarized ones and now I'm on board for the add-ons.
Unfortunately, I still think the blue is darker than I was hoping for, but it's still pretty good. At some angles you can see the blue more, and the electric legends make them pop. Yes, I still have qualms with the capitalized bottom row, but that's what this IC is for.
Rather than comparing this blue to the SA version, I realize this is more of an inky dark blue than the sun-dyed blue of the SA iteration. But as moistgunando astutely discerned, the name "Penumbra" couldn't be more fitting here, then--"pen"+"umbra" (i.e., dark ink).
Interestingly, I turned over some of the modifier caps and could see some bleed of a lighter teal. I wonder if the color just got a bit darker the more saturated it became as it filled the keycap molds.
Definitely pleased with the set, and eager to get this IC turned into a GB!