When is ghusl obligatory: Is it after the usual period of menstruation, or after the brown discharge that follows menstruation stops and the white discharge (al-qassah al-bayḍā’) then appears?

A woman becomes pure by dryness. If she sees dryness—meaning, if she inserts a cotton swab and it comes out clean with no trace of blood, yellowish discharge, or brownish discharge—then she has become pure and must perform ghusl. She should not pay attention to any brownish discharges she might see after that, based on the statement of Umm Atiyyah (may Allah be pleased with her): "We did not consider the yellowish or brownish discharge after purification to be anything." However, any yellowish or brownish discharge that is connected to blood is considered menstruation.