One of my favourites is this floral design painted entirely in black over a duochrome polish. In real life the effect is stunning, but sadly on camera it was super hard to pick up. As a result the photos kinda suck....even running around the house for 20 minutes taking snaps in every light condition at all angles wasn't enough to capture it.
I used a Stargazer polish for the background, number 307, with black acrylic paints for the details. I love the finished look but if you're not into such dark colours, try painting the flowers in white over a lighter background.