What is makeup oxidation? It’s what happens to your foundation after it’s been applied. A change of color occurs, making your skin look darker or more orange over time. This change occurs for a variety of reasons, such as the PH level of your skin, the natural oils on your face, or even the air around you. Makeup oxidation is a common problem for many women, especially those with oilier complexions or those that live in humid environments. Ingredients commonly found in foundation, such as iron oxide, are notorious for contributing to oxidation.