Brownells sells a welding rod that is supposed to match color wise with most gun steels. I have never used it so can't speak for how well it might work. If you pose your question on other gunsmithing forums, you may get a response from someone who has actually done it. I have used soft iron binding wire for weld up small parts and the color match seems to be very good with cold blue. On larger surfaces, small welding repairs may show up though. I know that if iron wire is used and then part is parkerized, there is a color change. I am assuming that you are doing the work yourself so the cost is not such a big deal.