This thread reminded me of something I noticed a few years ago - I have 4 x 410 Program Recorders (of course all of them need new belts but I digress ...). Three of them are fairly yellowed but one is not nearly as much. So for a quick comparison, I grabbed two including the one in question.
The one on the left was acquired CIB years ago from a collector and other than the dust shows no real signs of ever having been used; all the paperwork is in THE PROGRAMMER box it came in, including the thin cardboard inner box, the box for BASIC the cassette and box for STATES AND CAPITALS, manuals, warranty card, Atari Connection card, etc. The one on the right, by contrast, was acquired along with one of my 1200XL's; the 1200XL has a "PROPERTY OF MENDOCINO CITY SCHOOLS" inventory tag on it; I presume this 410 came with that one, complete with an aftermarket replacement white power cord and a service sticker on the bottom from an almost-certainly long-gone service company. Anyway, the one on the right was used and definitely NOT complete in box from a collector. As you can see, they are noticeably different colors. I strongly suspect, and my recollection tends to support this, that the one on the right is closer to its original color than any of my other 400/800-era computers and peripherals. I left the white Styrofoam from the boxed unit in the photo to provide a good white reference if anyone wants to tweak the photo to compare colors.