The patina on my little folder is indeed a mustard finish. I'd like to take credit for the technique, but, alas, I stole it from a very talented and generous bladesmith by the name of Michael Rader in Washington State.
The process is beyond easy. It works better if the entire blade is hardened; for some reason, where there is a differential heat treatment and part of the blade is left soft, the finish doesn't take nearly as well on the unhardened part. First, finish the blade to 400 grit or so, and clean the blade so there's no oil, fingerprints, or dirt. Next, using a tooth pick, q-tip, or small brush, apply little blobs of mustard to the blade. I use inexpensive French's yellow mustard, but I don't think it matters much, as long as it's a fine paste without whole mustard seeds. Leave some space between the blobs of mustard; you'll get those on the next pass. Hang the blade up somewhere warm and dry, and go watch tv or play with the kids. After a couple of hours, the mustard will dry and turn blackish. Clean off the dried mustard with soap and water, and then repeat the process until you're satisfied with the results. The pattern of the patina will depend on the shape and density of the blobs of mustard you put on the metal. I use the dull end of a wooden shishkebab skewer to apply the mustard, and I make little tiny round chocolate chip shaped blobs, maybe 1/16th of an inch in diameter. Getting a good, dense patina usually takes three to five mustard applications, and the pattern gets progressively darker and denser as you continue.
One thing about this patination process that I've never been able to figure out is that, sometimes it results in a rainbow sort of opalescence to the surface, which is really pretty. For me, sometimes this happens, and sometimes it doesn't, and I've never been able to figure out why or what to do to force that outcome. Maybe one day I'll run into a metallurgist who can explain that phenomenon to me. (And, if you somehow figure out how to make it happen, please let me know!)
Hope this is of some help.