Purple blue and black.

This little beauty of a nail polish is part of the Model's Own Beetlejuice collection, which for the record, you need. This one is the imaginatively named 'Purple Blue'. It's an incredibly pretty duochrome, and shifts from purple to blue (bet you couldn't have predicted that) depending on the light and angle. It was pretty hard to photograph - ignore the mess in the background by the way - but on the thumb you can see a little bit of the colour shift. 

In order to spice it up a bit, I added an angled black tip to each nail. Easy peasy - I did it with cheapo Primark invisible tape, taped off most of the nail and painted a thick coat of black over the uncovered part. Seche Vite on top'll give you this nice shiny look and dry it all in 5 minutes flat.