What makes your boogers yellow or green (chemically speaking) and does it always indicate some kind of infection?

White blood cells: When body discharges (nasal, sputum or phlegm, ear, genital) are yellow or green, it's generally because large numbers of white blood cells are present, often with traces of blood. It often indicates infection, but not always. It actually means inflammation, and there are non-infectious causes. For example, nasal discharge from allergy can be the same color.