Yep, the spacebar on my Plus' keyboard and a few of my early ADB and extended keyboards all seem to be slightly more beige than the rest of the keys. I've heard somewhere that the plastic spacebars were provided by a different manufacturer than the one that made the rest of the keys, and it has a tendency to discolour more.

The spacebars on my AppleDesign keyboards seem to be consistent with the rest of the keys.