I tried something a bit different the other day - something I've wanted to give a go for a while now! I've seen this called a flip mani, or an Louboutin Mani - after the famous black shoes with the red soles.
I painted my nails like normal with the black Kate Spade Nails Inc polish - two coats. I then used a barry M red gelly shade to paint the underside of my nails. This took a bit of precision - it was so hard to get it far enough down without getting it all over my fingers - it took a while, and a bit of careful cleaning up! It also took a few coats to build up the red, as the black coming through from the other side of the nail darkened it a lot.
I was really happy with the results though - they looked so striking!