I use paint pigment (it is about as fine as baby powder) mixed with gun oil. I personally use Nyoil from brownell's, but I think any lubricating oil will work. The paint pigment I use is Burnt Sienna from the "old milk paint company", it's a burgundy/red that shows up well on any type of wood. Mix it up in a small jar and apply to the metal with a brush. I think it's a lot better than lamp black or prussian blue & it comes off the metal by spraying with gunscrubber.