There is often much confusion about the difference between mica powder and pigment powder. Both can be mixed into and used to add colour to resin, soap, candles, and translucent polymer clay. They are both colourants and can be used to colour the surface of polymer clay. But they are not the same thing. Here's the difference.
Mica is sparkly. Pigment powder is matte. They behave differently. But online listings are often confusing and they're sometimes sold mixed together in the same packet. And that can mean the powder you bought won't work as you expect.
Mica is a natural stone mineral with shiny flakes. When mica is ground into a powder, you get mica powder. Mica powders are therefore sparkly, sort of like very fine glitter. They're used to give a metallic or shimmery pearl-like effect. They come in many colours. Pearl Ex is a popular brand of pure mica powder. (See a colour chart of all Pearl Ex colours here.) While mica powders do often have colour, they're not optimal for colouring things because their main purpose is to create sparkle or shine. They won't create a solid wash of bright colour.
Pigment powders are ground-up colours, sort of like powdered coloured chalk. Pigments are the actual colours themselves and have names such as ultramarine blue, cadmium red, yellow ochre, and titanium white. There are also artificial pigments with names like Pigment Blue 15. Pigments are what manufacturers use to give paint its colour. They are not sparkly. Pigment powders are excellent at colouring things (this is their job, after all). Pure artist's pigments (pure colour) are fairly expensive, so pigments you buy from cheap sellers online are usually mixed with fillers (such as chalk) that make the colour go farther. (For the nit-pickers among us… in recent years the word pigment occasionally refers to mica and synthetic mica particles. Therefore it can be said that mica is a type of pigment, but pigments are not mica. But in reality, these materials are two different things and are very dissimilar materials.)
Confusion happens when sellers of craft materials try to gain buyers by using confusing keywords. You often see listings in online marketplaces (like this one on Amazon) where the terms are used rather loosely. In this case, I'm not exactly sure what you'd get. You might get pigments, you might get mica powder, or you might get a mixture. And while we're at it, these colourants are not a dye, contrary to what the listing says. Pigments and dyes are completely different things, as explained in this article.
Both mica powders and pigment powders are used in the cosmetics industry. In fact, a combination of pigments and mica powder is used to create eye shadow, blush, and powdered foundation makeup. These can be used with polymer clay. You can read about Using Eye Shadow with Polymer Clay here. Cosmetic supply companies are excellent sources of both pigments and mica powder. But choose a reputable company to be sure of exactly what you're getting.
While both mica powders and pigments can be used to colour the surface of polymer clay, they do behave quite differently. Knowing which material you have will help you know how to use it in your clay project. These are both incredibly versatile materials that can be used in so many ways beyond adding rosy cheeks to a cherub. For example, about a third of the veneers in my 100 Days Project are done using mica powders and pigment powders. Here's an example of a veneer using only pigment powders, mica powders, and a stencil.