Here is the background for my next acrylic painting which is 12"x24". I got this look by painting the background black and brown. After that dried, I used a sea sponge to dab on a mixture of colors: greens, grays, browns, off-white. I sprayed water on it to allow the colors to mix. You have to be very patient and not disturb the paint as it dries. Let it do its thing and this is what you get.