Bupaje - something to play with, via my brother (the painter in the family). One exceedingly cool thing I've seen him do for a number of his pieces, and always thought would be exceedingly cool for animation backgrounds. If you want linework as well, sketch out the lines in a waterproof india ink. But not necessary. To core of it requires oil (
not regular) pastels and plain old crayons. Sketch out the colors in the oil pastels/crayons. When the sketch is done, blend out the color fields with a brush and some paint thinner (mineral spirits) to get a more "watercolor" kind of look, with more control. The oil pastels will tend to be a litttle more dense, the crayons a bit more transparent. Depending on the amount you work the pigments with the thinner, you can go from a straight, as layed down look to very washy within small areas. Very cool.
Another bit that I like a lot also involves crayons Sketch out areas you want to have that "crayon" look (with a large background it can work great for colored outlines, but can also be used very lightly for textures). Finish the piece with watercolors, and the wax in the crayons works as a resist and the watercolors won't touch the crayon. If you use a heavily textured paper, you can have very sketchy spots of crayon colors that will be completely surrounded by, but never actually infringe on the particular spots the crayon is at. So you have typically broken up crayon lines that are seamlessly surrounded by color. Very hard to do any other way if you are physically painting.
And if you are doing watercolors, get gouaches, too. I like to do roughs really broadly (and way washy) with watercolors, let it dry, and then go back and add detail with the more opaque gouache.
And don't forget about cel vinyl for backgrounds. It's hard to force it to take brushmarks, and hcan be a great medium if you want a smooth, graphic look. Better that acrylics, IMO.
I'll see if I can have my bro check out this thread and throw in some input. I know he's thought about this kinda stuff a lot, and he knows (or came up with) more non-standard techniques than you can shake a stick at. A great painter and if you ask him about composition and color you might get a textbook that should be written (I've tried, I really have).
And and extra aside - I figger you probably have, but if you haven't played with ArtRage
http://www.ambientdesign.com/artrage.html you really should. I like it a whole lot.
I am not depending on it yet, but hope at the end of the 18 months (I'm only 2 months into this exploration) that I'll be able to get in to a creative field at some level -not sure that it would be background painting, might be storyboarding, writing or who knows what.