In reply to beaver_9.

Winner, winner. Some of the guesses made by others are also good uses for crayons, just not the one I executed here.

Some of the custom keycaps for mathematics and Greek characters on my Olympia SM3 were originally etched in and then filled with either wax or paint which has since disappeared. (The other keys seem to have been double-shot.) White crayon seems to have not only filled the gaps, but does exceptionally well for a color match. Given the depth of the etching of the glyphs and the wax, I suspect they'll be good for a few more decades of use, and it has the added benefit of being an easily reversible restoration technique if it doesn't work out. Wiping off the excess crayon outside of the characters was also a straightforward and easy process.
Here's the before and after photo:

cc: u/IrmaBecx, u/OalBlunkont, u/guneeyoufix