Can't say for sure why the color is changing. I have seen chocolate dogs that shed out a light color of brown and a darker color will replace it. Usually this happens for a couple of reasons.
1. a change in diet from good to bad or bad to good will change the coat appearance.
2. Changes in living conditions from outside to inside.
I just can't say for sure. There is the possibility that the dog is not a full blooded lab. Don't let that bother you, if the dog shows potential run with it. Keep training and have fun!!!