I found the following imgur album a good representation of X-keys clear caps:
http://imgur.com/a/gTpfd
Even a tight layout of 18mm works okay:
without caps there's notable hotspots on the clear caps (could be the photo/camera settings):
In the dark:
From this angle you get the idea of the "switch mounting panel"
The black painted keys with translucent "negative" labels look the best IMO.
Apparently RS used to stock these, I can't find them anymore
Back to X-keys, result isn't too bad with mostly black legends:
Looks like quite a bit of colour bleed; fine for bling RGB where you want a rainbow effect, less so for individual indication.