Internal reflections and modern coatings can alter the contrast of a lens. This can be the cause of what you call "grey whites" compared to old lenses and "white whites". It also effects the whole color range, meaning acrylic colors versus antique pigments.

Today's lenses can be made essentially perfect, with sharper pictures and crisper colors. And this is what make old lenses popular, because of their character.

Since film is infinitely variable, unlike digital with its defined 'steps', when combined with older lenses we get unique images that digital can not create.

Therefore, old lenses which were thought of as imperfect in their day, are now valuable because of their flaws.