Honestly? I think it isn't intended as symbolic a lot of the time. I think that whenever it is used in reference to yellow fever, it is literal. That's for the most part. It refers to the way the fever makes the skin jaundiced. However, once the idea of the fever is introduced, the color yellow takes on a host of symbolic meanings. The yellow fever plague was devastating, and so it—and the death that followed—seemed to be everyone. Yellow came to symbolize death, suffering, fear, helplessness, hopelessness…

Greg