The paranasal sinuses are air-filled cavities that are lined with mucus-making cells. The mucus serves to keep the nose from drying out and to trap contaminants before they reach the lungs.
Mucus (”snot”) is made of water, proteins, antibodies and salts. Its color is an indicator of health:
CLEAR indicates healthy.
WHITE is thick mucus indicating sinus congestion.
YELLOW means that infection-fighting cells are battling a cold.
GREEN indicates that your immune system is fighting a bacterial infection.
RED is blood in the mucus, possibly from broken tissue in the nose.
BROWN could be blood or an inhaled contaminant.
BLACK, unless you're a smoker, could mean a fungal infection.