Precisely! Drops of mustard, with some apple cider vinegar mixed in for good measure (not sure if necessary but I like the results so I just always add it). Applied with a toothpick. Then let it sit for about a half an hour, rinse, and repeat. It's the overlapping from the second coat where the colors really come out. There's a pretty good tutorial on YouTube where a guy does it on a Opinel.
For the main blade I just let it happen naturally with a couple weeks of fruit cutting. I pretty much do this with all of my multi-blade carbon steel knives, letting the main blade patina naturally then forcing one on the secondary blade.
I know this is not what you were asking, but for anyone interested here's how I put a patina on the brass flashlight. Make a mix of apple cider vinegar and salt in a container. Then use metal screen (or something similar) to cerate a shelf where the item can rest above the liquid. Put the item in, close the container and let the fumes from the mixture do the work without the liquid ever touching the item.
Anyhow, some more pics of blades I've done this to: