Here is a miniature that I finished quite a while ago, but I never got around to posting him. He's from Studio McVey's stunning LE resin line (shame we haven't seen anything new from it for a while).
I painted him up for an RPG, and I think he came out rather nicely. For most of my minis, I prefer to use a black undercoat, but I decided to try white on him instead, to see how I liked it.
I discovered that I didn't like it at all. While the painting over the white isn't significantly different from painting over black, I found that it was more difficult to tell if my composition was working as I went- the bright white of the unfinished areas made it very hard to tell. Conversely, with a black undercoat, I can usually tell whether I've highlighted an area light enough and if my overall composition is working out.
The advantage that you get from a white undercoat is that you can more easily see the detail on the mini before you paint it, but that's not something I struggle with. While some people claim that your colors will turn out brighter if you're painting over white or black (yes, I've heard this claimed about both) I find that if you're doing enough layers to get a solid base coat, there's no way to tell if you used white or black to start from.