Phlegm is a thick liquid secreted by the respiratory mucous membranes, especially in excess during colds. Its composition varies depending on factors like climate, genetics, and immune system status. The color of phlegm provides clues to health conditions - white/grey indicates upper respiratory infection, green/dark yellow signals bacterial/viral infection, brown results from smoking, and bloody or pink phlegm requires medical attention. Excess phlegm can be caused by vocal abuse, smoking, respiratory illnesses, allergies, pollution, and certain diseases. Home remedies to reduce phlegm include drinking fluids, using saline sprays, gargling salt water, and avoiding smoking and secondhand smoke.