I'm a big fan of the painted-on background. I've only done spraypaint in the past, but am going to try brush-on latex next.
Maybe take a stroll through the paint section of your local hardware/paint/home store. sometimes they have discounted cans that were returned/problematic/ihavenoidea for really cheap. Usually it's typical household colors, various shades of beige and off white, pastel blues, etc., but sometimes there is something interesting in there.
Also check and see if there are any hazmat recycling/collection centers in your area, I remember one near me was offering free latex paint (I think they just mixed all the excess they collected, but I'm not sure...), but it's likely to be in the common shades.
If you are really hard up, you could probably just take some oil (cooking oil, mineral oil, crisco, vaseline, used motor oil, etc.), smear it on the glass, and then stick even a plastic trash bag or something on it, work it a bit with a credit card/stiff scrapertypethingy, and get a cheap (free?) background...(I haven't tried this, and wouldn't recommend it...)