John,
I like this one because it works will all colors of animals (good for people, too).



Black is my favorite because you can do this with it.



White, of course, for high-key effects.

Basic blue works well either as a solid color or mottled -- we mix in some washable white spray paint at Xmas time to give it a bit of a snow effect.



Afterwards, we just wash it out and we're back to a nice stone-washed blue backdrop. We've done the same with a rich red which we dyed ourselves using white muslin. There are a slew of other mottled color variations but I think keeping it simple works best. I'd add in a charcoal/gray backdrop which works well for dogs with great color -- Irish Setters for example.

Start with some basics and you can always add stuff later after you've seen some examples of them in somebody's gallery.

Jim