Maybe they used mineral oil and then dusted the hat with some lighter-colored Fuller's earths. Then the Fuller's earth would stick to the mineral oil and the rest could be blown or whisked off the other parts of the hat.

I'll go with the Browning Hi-Power. It's reliable, accurate, durable, carries 13 rounds in its magazine (+ 1 in the chamber, if you so desire) and takes far less time than 1930's-era revolvers to reload (no speed-loaders yet), giving you another 13 rounds to use in ridding the world of sadistic, meg...