Green or Dark Yellow Phlegm
A thick and dark yellow phlegm may be a sign of a viral or bacterial infection, sinus infection, or lower respiratory tract infection. Typically, this occurs when the immune system sends white blood cells, known as neutrophils, to the area of infection. These cells contain a green protein, which, when present in large quantities, turn the mucus into a greenish hue.
A 2011 study published in the European Respiratory Journal, however, found that green or yellow phlegm does not always signify an infection. After some tests, researchers found only 59 of every 100 samples of green phlegm contained bacteria while only 46 out of every 100 samples of yellow phlegm contained bacteria. “Contrary to popular belief, having greenish mucus does not necessarily mean you have a bacterial infection,” Exline said.
Source: Sputum Characteristics