I don’t have that specific lens but quite a few other Nikkor lenses from the early 70’s to some of the last produced AiS lenses and notice that there are differences in the brightness and tones of the color inlayed in engravings unrelated to age - and I am talking about those engravings normally not subjected to direct touch and friction.
Environmental contamination can be a reason, but it is normally fairly obvious if a lens has been exposed to nicotine or similar direct pollutants. It tends to be uneven, affecting all cavities - not only the engravings - and can be smelled decades after the exposure.
These more minute and even differences I see on my lenses are more likely caused by changes in the paint compositions and the associated aging properties of the paint delivered by Nikon’s chosen subcontractors, perhaps in combination with environmental factors such as heat, humidity and possibly from storage in a container or cabinet emitting some residual chemicals evaporating from the cabinet finish.
I worked for years in an art museum and we were extremely attentive to the properties of materials used in our storage facilities, not only those materials in direct contact with artworks, but also wall paint, shelves materials etc.
I saw an example of a white object which had been standing for some years next to (but 15-20cm apart) a brownish box of some unknown plastic material. The white object had a significant yellowing tone on the side facing the brown box - but no discoloration on the other side.
This example makes me think that use of inferior plastic in the lens cap also could cause discoloration of the engraving.
