When I worked at Genicom, they had a printer refurbish department for "all
brands" and I saw various Epson, etc. printers going out that looked just
like new, and I saw them coming in with the yellow discoloration. I asked
one of the guys how they did that, and he said that they painted the covers.
The discoloration is caused by UV light exposure, which in turn comes from
fluorescent lighting. It won't happen under incandescent light, but all
offices have fluorescent lighting.