A few notes, and not to take ANYTHING away from what Max has offered so far. He has a wealth of knowledge and is actually responsible for improving my own painting in many ways.
-I use VMC Ivory for a couple of reasons. One because I dont want to use a pure white with my gold, as Max says. But really I don't care too much about this. I use it because it is a superior white that doesn't get screwy like most whites tend to by leaving a flakey finish. Ivory and VMC's other off whites are all incredible.
-yes, I paint mostly through glazes, or by a series of thin layers built up with a range of different tones to make a solid transition from dark to light. Basically, apply a mid tone, not diluted much. Then use progressively darker versions of the mid tone for you shadows, lighter versions for your hoghlights. These are all fairly diluted, though your last highlight of white or near white is almost not diluted at all. Shadows get a good deal of dilution. The colors added on to add depth, like purple with NMM gold, get the most dilution.
-I use the purple to both add depth as you imply, and contribute to the shadow transition as well. Meaning the purple makes the black even darker. A note about the black used for NMM gold. This is highly diluted as well, and the more layers you add the darker it gets. So be real subtle at first with a thin, barely perceptible layer. Then add more the deeper and darker you want it. So the purple can add depth, especially if you use it where the mini has pronounced curves or rounded areas. This may not coincide with areas of darker black, to aid in the transition as discussed. That's okay, in a few areas you may have purple without any or much black underneath.
-lately I've preferred a red brown gold. So instead of using the black, I will use GW Dark Flesh, and even glaze some more red or even green in place of the purple used for te other recipe. The most important thing you will find is how that base color looks on its own, with your shade over top of it, and with more and more white added. The last part is probably most important. Does it look shiney and bright when you start adding white to the base color?
More to follow later....