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Yellow phlegm in particular can scream infection. But, interestingly enough, the color does not come from infection. A yellow hue to the mucus is caused from white blood cells, according to Health.com. So, yellow mucus therefore isnât a result of an infection, but a result of the body fighting an infection. Coughing up yellow mucus therefore can be associated with battling the illness as opposed to being a symptom.
Firstly, if the phlegm is green, yellow or brown in color, it means that the body is in the midst of fighting an infection. Coughing up of dark brown phlegm could be due to smoking or inhalation of excessive amounts of dust. This results in the buildup of resins or tars in the body which are expelled from the body through coughing.