The secretly painted underside is a trend I've noticed in nails which has also crept it's way into all beauty and fashion. The accessories company Mansur Gavriel was made famous by their secretly colorful bags which have brightly colored insides and simple colored outsides.
To achieve this look you need to have a substantial "free edge" (the tip of the nail that is off the skin). Use a skinny detailing brush dipped in the color of your choice and poke the brush downward on the backside of the nail slowly filling it in. Allow it to dry and apply another thin coat. The color you choose for the front must be of an opaque blend so that the opposing color on the back doesn't show through the tip. Sally Hansen Miracle Gel™ is a great choice for this because the majority of the colors are creamy and opaque. This is a detail that's perfect for someone who likes to make minimal but cutting edge statements.
Products used: Sally Hansen Miracle Gel™ in “Pinky Promise” and “Rhythm & Blue.”