Bloo and Rusty have the right answer. Here is a link to a Youtube video posted by a guy who restores antique tools. He experimented to find a formula for an authentic japan finish. If you are really concerned with authenticity, it is probably possible to reproduce japanning. If not, then just use any modern black paint. Like Bloo, I have seen japanning on antique door locks and other old cast iron hardware. Maybe white pigment was added to the black japan paint to make gray. I can't imagine it possible to make any other color in japan paint?