If the forend is already finished, then you will have to strip it. The only way to really get a color similar is to have a piece of wood (walnut ?) that is close to the color of your forend and make samples of each stain until you get at least one (1)shade lighter than the finished piece, because the final finish should put on will be very close to the color of the buttstock.
It is very hard to tell you what to use without seeing the color of the stock. If you could take a high resolution digital picture of it and post it, someone might be able to tell you what to use.
Without a sample to use and if you have to use the forend for a sample I would stay away from annaline dyes, they penetrate too far. I like Watco for stocks and forends and for furniture. It is almost like Danish oil finishes, you can rub it in with 400 wet/dry sandpaper and the slurry will fill the grain and give you a very smooth finish to put your final finish of choice on or leave it alone.
