I would stay away from those. They are not a dye but a highly-expensive laquer that is not absorbed by the emulsion but sits on top of it. The examples I've seen showed obvious tell-tale signs of repair. You don't get that with Spotone or ink.
I've tried them too, for whatever they made them, at least it's not for retouching. I have three of of them: brown black, neutral grey and jet black. Anyone paying the postage may have them.