I'm kind of curious; are you looking for a, say, dark blue sky with stars painted onto it (à lá toothbrush splatters,) or are you looking for something that can be lit to give you day / twilight / or nighttime effect sky?
If you wanted stars but without the tooth-brush, I'd
guess that a small Japanese or Chinese 'bamboo' brush would be the way to go for painting the stars...
Now, if you want to be really funky about it, consider an alternative in blacklighting. Wildfire has some really neat paints that go on clear that respond to UV lighting... difficult to make work well for a standard home theater, I'd imagine, but something worth looking at. They're at
http://www.wildfirefx.com
Leo Kerr
Lkerr1@alumni.umbc.edu