Paul, you can try to mix a grey stain but I have yet to see one mixed that looked right. You could also play with ammonia, but I don't think that there is enough tannin in pine to make that work. You might try a product called Weather-It that is available thru model railroad shops. They probably will not have it on the shelf, but if it is still being made, they can get thru Walthers, a large hobby distributor. Whatever you choose, you should try plenty of sample pieces to insure that you get a good dry color match and you will probably want to add nail holes and other distressing to the wood you use. Good luck.