The newest Mini-Challenge at Unity Stamp Company is called Grounded to Space ... using an inky background to ground something that isn't on the ground (i.e. air or sea). I don't remember every consciously doing this technique before, so I had to give it a try.
The only Unity Stamp I could find in my stash that fit the theme was this Airplane from For the Guys. I decided to "ground" it by using white pigment ink on blue cloud stock to create a cloudy sky ... I used a triangle of sponge and a hand-cut curvy edge to create my sky.
When I stamped the plane on top of the clouds, the ink was lighter where it stamped over the pigment, making it look like it was trying flying through the clouds ... happy accident.