One of the few polishes I've got left that I've never done a swatch picture for, Rescue Beauty Lounge Bubblegum Punk (here over black):
collaboration with Refinery 29. It just looks like white iridescence, mostly, although you can see that over black it looks somewhat blue. You can build it to something close to opaqueness on its own, or you can layer it, and historically I have loved this as a layering polish. As KellieGonzo says at the link up there, this has sort of a cellophane quality to it. It's quite extraordinary.
That collab, R29+RBL, produced two of my favorite polishes ever - the other one is Pretty Gritty. Unfortunately, RBL is now out of business so unless you want to shell out serious money on eBay you're not likely to find this one.