A white undercoat will give brighter colours, but you might get these annoying white specks in crevices – places your ordinary paint didn’t cover, and where the undercoat shows in a most irritating way.
A black undercoat will give a more dull finish, and you’ll find it hard to cover it with yellows and reds for example.
You can off course undercoat with other colours. If you would like to paint a figure in mostly reds, then a red undercoat could serve you very well.My take on undercoats is a mix of white and black.
I paint the figures with white paint, usually Citadel or Vallejo (simply because I have them). You could use spray paints, but I prefer using a brush, mainly because the climate makes spraying outside impossible during at least half the year, also because I’m not very skilled with airbrushing and painting by hand will give me a chance to study the details of the fig.
I then give them a black wash, and there you have the best of both worlds. A light base with dark crevices. Easy to cover, no white spots, and instant shading for lighter colours with weak coverage. It also makes it easier for you to appreciate all the details of the figure, so it helps you in painting.
An extra plus is that you will see mould lines easily, as on the head on this zombie from Mantic. This saves you the irritation of finding those lines the hard way, when your figure is nearly finished.