I took some pure white comb out of my hives that was attached to queen cages. Those combs are several years old like yours. Now they are tan (lighter than a paper bag) from whatever aging process. It is interesting that there is some change without the bees.Three or 4 years ago I removed a large piece of pure white "wild" comb from a hive and have it setting in my shop. There have not been "little feet" or bees on them in years.
The comb is now a bit darker than a paper bag. Discolored by exposure to the air or does comb just "turn dark"?