For NTNA Challenge 3, sponsored by LeChat, contestants were asked to create a double nail design, with one design on the underside of the nail and a transparent design layered on top. Here is Ashton Harlan's entry. See all 10 entries here.
In this video from the NAILS Troubleshooter series, MaeLing Parrish shows how to do some creative color backgrounds for a black crackle polish top coat.