Dab some powder pigment (like eye shadow) above the cuticle before the polish dries and blow the color onto the nail. Apply a top coat and admire! I will have to try this with all the Mac pigments I have!
HOW-TO: First you paint a regular coat of dark polish all over your nail. Quickly put a few small dots of lighter polish on your nail while everything is still wet. Swirl them together to create a marble effect with either a fine brush, a toothpick or a needle. Easy! The more you mix them together the more subtle the design will be, the less you touch it the more dramatic it will be. THIS IS SO COOL.