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

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