In my class, we were taught to do it a couple of ways (these
only apply to Sterling, unfortunately):
For a really dark black, heat the piece gently and apply the
liver of sulfur. If the liver boils off, your piece is too hot
and the black will flake off eventually, and you should do it
again with a softer flame. Then, just rouge or ZAM the raised
parts with a harder buff (a soft buff will get in the cracks and
take off all your hard work ) You can also heat the liver
itself on a hot plate in a heatsafe container, and dip the piece
into the hot liver. Both methods are followed with a rinse.
For blues, greens, reds purples etc. you can use the hot dip
method. Run a very hot stream of water in the sink, dunk the
piece, then apply liver (the piece should go yellow) and rinse in
the hot water. If you do this enough, you get a rainbow effect,
or a solid bright colour, finishing with a rinse.... you have to
wax/lacquer it right away, though, or the colour will eventually
darken into black as it oxidizes naturally in air.
Hope this helps.
I've seen some WILD colour effects with the hot water method.