5,6-Dihydroxyindole is an indole derivative crucial for eumelanin biosynthesis, responsible for pigmentation in skin, hair, and eyes. With two hydroxyl groups, it acts as a strong antioxidant, scavenging reactive oxygen species (ROS) and reducing oxidative stress. It oxidizes through the Fenton reaction to form melanin, providing both pigmentation and protection against oxidative damage, especially in inflammation.