Here is a technique I did a while back with Colorations paint, but have also done it with Twinkling H2°'s and watered down acrylic paints. Just be sure to have some metallic colors in the mix.
Use a few colors that blend well together and put randomly on heavy cardstock or watercolor paper and spray with water. Take plastic wrap and set on top making sure you have plenty of wrinkles in the plastic wrap. Set aside to dry completely. Remove plastic wrap only when completely dry and you will have a beautiful background.