Does anyone know if it is possible to have keycaps that show a different legend based on the color of the switch led?
So if I change to layer 1 and all the switch LEDs turn red, then a specific set of icons appear on the keys. And, if I go to layer 2 with green LEDs, it shows different icons…
I am guessing that it would involve some sort of color filter screen embedded into the keycap to only allow specific light waves through in different shapes?
Does that exist already?
Not exactly what your after, but similar.
I think it could be possible with paints or injection-molding shots that filter out a single color of light, but are otherwise dark enough to not be distracting when unlit. Finding the exact materials that have the right optical properties would be difficult I think. It would also be limited to red green or blue light because RGB LEDs only produce light at 3 different peak wavelengths and produce different colors by changing the intensities of each diode. The lit legends would have to be at different locations as well.
I’m by no means well-educated in this subject, so take this with a grain of salt. But I think it’s a really cool idea
Ain’t there a keyboard with all keys similar to the elgato stream deck 
that should work for you.
I think that’s what Andreas was linking to before, the Optimus Maximus and Optimus Popularis keyboards have an OLED screen in each key. I haven’t seen them for sale in a good while though.