When I first started creating acrylic marbled papers in 2007, I tended to put patterns/colors all over the paper uniformly. Now, feeling emboldened about seeing how far I can push this art form, I'm "free styling" my technique in hopes of creating a happy accident. For example, I randomly drag my paper through the marble tray letting the color skip over sections of paper, creating some fresh white of the paper. It is in these "openings" that I can later develop some subject matter.
I've also started playing with my color mixes (just as I do when I'm painting in acrylics or watercolor). In the example I've provided here ("Walking After Midnight"), I managed to create my own version of indigo by mixing up mainly pthalo blue, plus a touch of burnt sienna and napthol red. I tested the color on a piece of paper first. If my color got too muddy, I just added more blue.
I like the monochromatic blue/green of this piece, with just a hint of magenta in the mix (I also collaged in a magenta moon for balance). This is still a work in progress, but I think I'm very close to being done.