For those of you who have read Alan Moore's splendid The Killing Joke, you may recognize this TS-138 as an elaborate pun on the origin of the Joker.
For everyone else, there was an extended acid bath involved, and a little high voltage.
It left the surface of the titanium with a velvety and slightly uneven texture.
I remember reading up on the mathematics of anodization years and years ago, but the long and short of it is, the more 3D the surface is, the more fractalized oxidation you get bending lightwaves and the truer color you get. But there's a tradeoff, because matte surfaces don't look quite as perfect as polished ones. At least, mine don't, yet.