Why do mucus turn green?

Mucus turns green primarily due to the presence of white blood cells, specifically neutrophils, which contain a greenish enzyme called myeloperoxidase. When the body fights an infection, these cells accumulate in the mucus, releasing the enzyme and giving it a green color. Additionally, the breakdown of cellular material and bacteria can also contribute to this color change. This green mucus often indicates an immune response to an infection, such as a cold or sinusitis.