Probably marker pen. I presume there was some need to remember to use those keys for whatever data entry type task it was used for, and either an instructor or the hapless office monkey highlighted them!
There's something similar at work, but using stickers, on my AT:
Maybe so the printed instructions for the worker could say to hit red, green and blue instead of the names of the actual mods? Once I get that keyboard from my brother, those stickers are coming off, of course. Better not be nasty residues…
Relax. 
