I mostly photograph my subject with a neutral background. I prefer the large 8'x12', pearl gray material with collapsible supports.
In composing the actual painting, this neutral background allows me the flexibility to add objects or landscapes that I've photographed separately. And it is much easier to have a strong color statement in the foreground and on the subject - not the background.
I also have the smaller background...but seldom use it as it "is never big enough." It is dark red and I sometimes use it as a drapery prop.
The reason I do not use black is that I want the light to "do its thing" and this doesn't work with black. I do not recommend black because it tends to eliminate the "atmospheric effect" that I get from the neutral gray. However, I always include a black object somewhere in a painting...but I really prefer it to not be a background (dark yes, black no).
I wrote an article about photography at
http://kcwells.com/technical2.htm and included info. on this particular gray background...
Hope this helps!