The one thing not mentioned as an environmental contributor to stock darkening is tobacco smoke. I helped a friends widow liquidate his muzzleloader collection, a few years ago. One of the guns he had was an over and under Beretta replica. He had been very proud of it and had it hanging on the wall in his living room. He, and his wife, both smoked like chimneys. When I got the gun it was quite dark. A lot darker than others I'd seen, and the finish seem a tiny bit tacky to the touch. I decide to take it apart and give it a good cleaning. When I slid the barrel band forward, I found the original finish was about four shades lighter. I wasn't worried, because I had dealt with the bullet proof sealer the Italian manufactures use before, and was sure cigarette smoke wouldn't penetrate that stuff. How wrong I was, a good cleaning only removed the stickiness not the darker color.
The only light colored antique stock I ever saw, that appeared to be original, was on a maple stocked barn gun. It appeared to have an old coat of shellac, or varnish, on it that was lightly tinted( or just darkened with age) or maybe it was tobacco smoke who knows.
Hungry Horse