This might be worth splintering off into a separate thread, but ...
A friend of mine had a neighbor die recently and the widow asked my friend to go through her husband's gear-filled techy basement, take anything he wanted and help get rid of the rest. None of the hardware was Atari unfortunately (he did have a very cool Heathkit machine that my friend took), but he also had toyed with the idea of buying a 400 around 1981/82 or so. There were a few brochures my friend sent me that have lived for 35 odd years in a file cabinet. Here's a quick photo showing all three. It looks pretty clear that Atari was not themselves terribly consistent in how they portrayed "400/800 Beige" (See the top brochure and compare the early-model 410 with the 400 next to it) and look how gray/white the lineup looks in the bottom photo.
Frankly, unless someone like Curt or Tim Lapetino has uncovered preserved, properly white-balanced color advertising artwork photos, taken before pasteup and layout by the ad agencies, we'll never really know the exact specified "shade" of plastics intended. And the reality is, those of us who lived through those years are remembering back through the mists of decades, those memories colored (literally in this case) by the machines we have at home around us every day now as hobbyists.
(Personally, my fuzzy memories of getting my Atari 400 in '82 and 800 in '83 seem to jibe with the colors/tones in the middle brochure abut who knows?)