A doctor should be seen when phlegm is yellow, green or bloody, according to WebMD. A doctor should also be seen when the cough does not go away.
The cough is the body's natural way to clear the airways of any mucus. When the phlegm turns a yellow or green color, there may be a serious infection that needs to be treated. Bloody phlegm is also a sign of a more serious issue. If the cough is accompanied by a high fever or wheezing, it may be more than just a common cold. The cold may have advanced into bronchitis. Asthma can also cause a chronic cough.