Outline nails! .. or, border nails? Not sure if anyone has come up with a definite name for these yet, but I knew when I first saw them that I had to try it! Some people say to do this by doing a base of the outline color, then doing the main color over that, making sure to leave a border. I just painted my nails the main color then did the outline with a nail striper. Wasn’t too difficult, but I’m guessing using a nail art pen might be easier.
Sally Hansen Mint Sorbet, Kiss Nail Art Paint in Black, Essie Matte About You