I don't know what grit you sanded the stock to, but I'd suggest 220 grit at least, and maybe 320 if you are going to use something like the Minwax Polyurethane. Their Fast Drying Polyurethane in Satin looks real good on Walnut. If you are going for an oil finish, I'd go as high in grit as 400. And after that you should raise the grain with water at least once and up to 2 or 3 times depending on what quality finish you want. That said, about the fastest finish that looks good and is durable (that I have in my inventory of finishing stuff) is that Minwax Fast Drying Poly. I'd put a screw hook in the butt end of the stock and hang the stock and spray the poly on it in thin layers. You don't want it to run. If it does run or sag, a rag wet with Mineral Spirits will wipe the run, but you run the risk of 'uglifying' it. So just go slow with thin coats and try your best to avoid the run. Practice on something else first.
As for the color, the Minwax won't darken it. The oil finish will darken it.