Is it obligatory to wash undergarments if the yellowish discharge that women experience reaches them, or is it permissible to leave it and pray?

If the yellow discharge is semen, then the preponderant opinion is that it is pure (tahir), based on what was narrated from Aisha, may Allah be pleased with her, that she used to scrape it from the garment of the Messenger of Allah, peace and blessings be upon him, and he would pray in it. If the yellow discharge is connected to menstruation or occurs after it, then it is impure (najis) and is not considered part of menstruation. One must perform ablution from it, but it is pure in the sense that it does not necessitate washing what it touches.