These cells contain a greenish-colored enzyme, and in large numbers they can turn the mucus the same color.
But "you can have perfectly clear mucus and have a terrible ear and sinus infection," Kao says.
Drinking milk may also make some people produce more mucus.
Kao says that's due to gustatory rhinitis, a reflex reaction that's triggered by eating.
Decongestants also have side effects, which include dizziness, nervousness, and high blood pressure.