It's probably fine. I've been shooting pinhole cameras with paper negatives. Since the cameras are unreloadable in daylight, I end up developing just a sheet or two often rather than saving them up to go on darkroom sessions. I keep the 1+3 working solution in a pickle jar between shots, usually with at least a little air. It turns a deep brown and starts taking a bit longer to develop, but it lasts for a very long time. If I was doing serious printing, I might notice my contrast shifting a little bit or something, but for my purposes I don't really care for pinhole negatives. It always develops to deep black and my exposure times don't change. The only thing is I start noticing it takes longer for the image to come up and then I leave it in the developer proportionately longer. If your stock solution is only slightly brown it is probably perfectly fine. As long as the image pops in a timely fashion I would consider it good.