Sallow means unhealthy in appearance––often yellow in color––and is almost invariably used to describe someone's complexion. His smile was as engaging as ever, but from his sallow complexion, I knew he was sick.
Usually we say that when someone is sick they turn "green." You could also say their skin takes on a sallow look, and you'd mean the same thing. Sallow also describes the way someone looks if they have lost blood or are pale from fever.