Must one wash underwear if a yellow stain is found after washing it with soap, while doubting whether it is semen?

The questioner is not required to perform ghusl (definition) (ritual bath) nor wash the area unless she is certain that the trace is from semen. This is because the default assumption is that it is not semen, and things remain according to their original state until the contrary is confirmed. This is supported by the hadith (definition): "He should not leave (his prayer) until he hears a sound or finds a smell."