Only the 1996 Pluses. The entire Apple line switched to corporate Platinum by 1997, including the 512Ke. Nevertheless, the beige plastics do in fact turn noticeably yellow, or orangey-brown depending on the extent of their exposure to UV and other elements. Check out the Retrobright threads – the before and after make the difference quite evident. The current shade of most of the early Apple products is most unappealing, yet the beige color Apple used was actually quite nice for beige, particularly the warm, somewhat taupe-hued, PMS 453 used on the first Macintosh. It's nice to finally have a way to be able to see it again.
You really don't notice the yellowing as much since beige is sort of a yellow-derivitive to begin with and of course they are all that color now. They worked hard after the Lisa to come up with a color and plastic formula which would resist the disastrous color change which afflicted the Lisa, some within a very short time, turned a horrible shade of Orange. The Macs age gracefully in mild office florescent UV and for that we can be thankful, as I doubt they would have the same appeal otherwise.