(from wikipedia)
Director Nicholas Meyer wanted the Enterprise to feel grittier and more realistic for Star Trek VI: The Undiscovered Country (1991), but realizing that vision was limited by the need to use existing sets. Cinematographer Hiro Narita changed the clean, bright look of the bridge from The Final Frontier by lighting it differently in The Undiscovered Country.
My interpretation has always been that the enterprise is getting pretty run down by that point and the wear is starting to show - we see this wear happen to plenty of other Federation stations (DS9, Starbase 80, etc.) so it makes sense that in the lore such a venerable ship would be starting to show it's age.

all 17 comments