Paint the entire background, however you like.
Draw templates in various shapes and sizes on a piece of paper.
Cut them out and trace them onto your picture. They can even overlap each other.
Paint the shapes any colours you like.
Bring out your inner artist and make abstract art with acrylic paint! You don't have to be a pro to make beautiful pictures. Acrylic paint is water-soluble, opaque and quick-drying, so if something goes wrong, you can just paint over it. Try brushing on the background and applying it with a sponge to see what kinds of effects you can create.