That's easier to do when you're building than on a gun already finished.
I'd scrub the wear areas gently with some 4-) steel wool (VERY GENTLY) or a scotchbrite pad to lighten the color 2 shades where a gun gets rubbed a lot- under the forearm behind the ramrod pipe where you hold it with the left hand or would rest it on a rest, also the edges of the lock panels, the wrist where your right hand is on it when handling the gun, up to where your thumb is when firing, wherever you hold the gun at rest. Normal wear points. Then, assuming you don't want to mess with the finish, you might take some beeswax, mix it with soot, melt it down and let it set up. Then rub it into the areas that normally collect dirt- the buttstock behind the wrist (which you lightened), the lock mortise, any cracks or places where the inletting is imperfect. Then out on the forestock, where a gun seldom gets handled, rub some of that sooty wax. repeat this every time you use it (the sooty wax treatment in specific areas) and in a month or two you may be pleased with the results.
If that does not give you what you're looking for, some analine dyes like leather dyes will actually penetrate some finishes, and you might risk darkening the stock in "low wear" areas using a dark brown stain. You'd need to be good at feathering it in and your hands will never be the same again unless you have rubber gloves.