Discolored / "green" vulcanite is caused by a chemical reaction. Usually called "oxidation" or "rust" by the general public, and known as "redox" by the science types.
The chemical reaction alters the material at an atomic level and cannot be reversed. (Theoretically, yes, in a laser-equipped multi-million-dollar laboratory, one molecule at a time, the same way lead can now be turned into gold, but nothing remotely practical under real-world conditions, never mind a pipe workshop).
So: The only way oxidized ebonite can be returned to its original color is to physically remove the discolored surface layer by scraping, sanding, or chemical means until unoxidized material is exposed.
And while chemical dissolution works, it is wasted effort if a smooth finish is desired because it leaves behind a pebbled, textured surface that must be scraped or sanded back to level anyway. (Vulcanite is like super-tiny-bead black styrofoam... dissolve away a finished surface, and what's underneath is rough.)
The end.
There are no secret methods or secret chemicals that will simply "make it black" again while leaving it shiny & smooth. A fact that is endlessly disregarded/ignored by the PipeWorld's patent medicine salesmen.
PS --- You definitely CAN "paint" it black again with ink or stain or dye, a method that works fine on objects like old radio cabinets or picture frames, but if stain is applied to pens, telephones, or pipe stems---objects subject to handling wear---all you get is a mess.
), and too soft for control.
