UV light can cause a breakdown in a lot of old finishes, and affect the color. Even things like humidity, air pollutants, etc, can cause changes. That is why the have all those additives in finish these days.
Darkened wood, under the finish, is generally caused by oil. This mostly shows at the front of the grips on stocks, and the cure is bleach. The cause is people over-applying oil to a gun, and when stored upright, it drains to the rear of the receiver, and soaks into the end grain of the stock where it meets. That can also cause changes in the wood, and cause it to split. When finishing/refinishing a stock, I always apply at least one coat of finish to the end grain that mates with the receiver to seal it.