More info about the rifle and what you want to do please.
Any rifle stock will change color over its lifetime, you won't change that without removing the old finish and reappluing a new finish.
If the stock is crudded up with grease or cosmoline, you can remove that several ways. One way is to heat the stock until the grease softens and you wipe it off with a soft cloth. Or, use a solvent that will dissolve the greaee and you wipe it off.
Most of the time, the change in color is due to the finish that was put on originally. If you use a restorer (like Formbys) you can remove the old finish and leave the patina behind and then put on a new finish (tung oil, boiled linseed oil, wax, etc.) to preserve it.