Having used acetone to create open spaces, I painted clouds and flower petals inside them. Check out the video. The full piece is here.
This is how I sometimes make empty spaces in the background. The background is a white acrylic ground, on panel, that had dried and cured; over the top are two solutions of different blues mixed with water and polypropylene glycol; the heavier of the two blues had settled to the bottom.
The finished piece is here.