To ensure that C15 is irritant-free, Paula’s Choice doesn’t formulate with dyes to ensure uniform color so there may be slight color variations with this product. Visit the Help Center for more information on C15 Super Booster’s color. Vitamin C serums with ascorbic acid, including C15 Super Booster, darken over time once the bottle is opened and used due to ongoing exposure to air and natural light. This is completely fine as long as it's used within 3 months once opened.