Would it be best to paint over white fondant or dark chocolate fondant? I'm thinking the chocolate but not so sure... I'll be using luster dust and vanilla to do this... TIA!
It depends on the dust. If you're using Wilton, you're not going to get gold from either white or brown...you'd do better to use dark ivory fondant, or yellow, depending on the shade of gold you want.
Ditto on what Texas_Rose said about the brand, but if you're using another luster dust (other than Wilton!), you can get pretty good coverage, depending on what shade of gold you want and how much dust you put in your liquid. Take a look at the very last cake I made, a purse cake. All the silver rings ("holding" the straps n place, for example) are white with silver painted on. All the little studs (the little balls all over the straps) and the zipper, as well as the chain holding the Coach tags in place, were all chocolate fondant painted with silver. Take a look at what I mean by intensity, and decide for yourself. HTH!
and you may want to use lemon extract or vodka over the vanilla since they have a higher alcohol content and evaporate faster.
Oh, and I've done gold over white and yellow and didn't notice much of difference between the two. Not sure how chocolate would look....