So I obtained some samples from GAHH, an OEM interior restoration shop, of both their Linen and Grey Beige. My '88 951 has the XP interior designation, which some have called one of the two names.
I also have a sample of the Linen from Automotion (the only color that they offer), and it is pretty much the same as the Linen from GAHH.
Here are the color comparisons, with the Grey Beige swatch being the darker of the two:
Door panels:
Rear Seats:
The seats in my car have been replaced by some from another car (don't know for sure which), so are close but have more of a red tint than the originals:
So in conclusion I think it is pretty obvious that Linen is too light of a color for the '88 and earlier cars, other than when the sun bleaches out the grey beige and makes your interior look like Linen. The '89+ cars are indeed Linen, however.