I recently asked my ID doc and one of my lung docs if yellow mucus always means an active infection. My ID doc said he believes that a change in mucus color during a flare-up is a bigger indicator of an infection than a specific color. The pulmonary doc said that yellow mucus can sometimes be caused by inflammation and/or an immune system response, even if there is no infection. She added that she knows of cases where yellow mucus was removed during a bronchoscopy and sent to a lab, and no bugs were found. Hope this is helpful. Peace to all.