My wife is a nail artist and active blogger/instagram poster. We recently upgraded from shooting her work in a light tent with a desk lamp to two 20"x28" 5 head soft boxes and the shots have been fabulous, with one major exception. Many of the new high end and independent nail polishes include iridescent pigments and are often referred to as "holo" polishes. In the right lighting, especially daylight, the polish has a dramatic rainbow effect. We have been completely unable to capture this effect with our studio setup, instead getting a flat silver glitter look, and I've been unable to track down any good resources on how to photograph this kind of effect. Below are a few links to examples of other instagram nail artists who seem to have worked this out. I was hoping someone could clue me in on what I'm missing and how I can make this work with my current setup or what additional equipment I need. Thanks.
Think of what happens when a rainbow forms: zillions of water droplets are scattered through the air refracting light. You see a rainbow because light from the sun is very directional and makes all the droplets refract light in the same way. Droplets located along a specific arc all scatter light of a given color at just the right angle to reach your eye. Droplets located along a parallel but slightly different arc do the same thing, but for a different color. The directionality of the light is the key to making that happen; if the light were soft and therefore coming from all directions, the droplets would scatter light in all directions and you'd never see the colors organized into bands.
You've got a very similar situation with the nail polish: there are thousands of tiny particles in the polish reflecting light, with some diffraction being caused either by the particles themselves or perhaps by the film of the nail polish. With a directional light, all the flakes forming a certain angle with the incident light and your eye will reflect the same color toward the viewer (or the camera). With a soft light, each flake will reflect light from many directions, and the result is a white or gray appearance.
You definitely need a directional component to make that iridescence show up. Experiment with the light or camera position to get the colors you want. You should also experiment to see if you can mix in a certain amount of softer light to make the skin of the hands look better without overwhelming the colors you want.