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. This word comes from Germanic roots meaning "dusky" or "dirty."