The way one of my customers does this on false walls, is to paint your background colour then flick paint sparingly at the background.At this point use a bird feather and comb over the paint at a 90 degree angle.(apparently pheasent feathers are best)
This is quite a skilled craft project. I recommend you buy a book specialising in faux effects with step by step instructions (any decent bookstore or library). I have a couple and they give good supplier lists for specialised equipment/paint etc. Oh and practice on a bit of old board until you have mastered the art. Good luck.