Why is it ruled obligatory to perform ghusl immediately upon seeing dryness, even though the cessation and return of blood, yellow discharge, and brownish discharge are normal and expected?

One should perform ghusl (ritual bath) immediately upon seeing purity, and this is the view of the majority of scholars. This is supported by the statement of Ibn Abbas, may Allah be pleased with him: "It is not permissible for her, when she sees purity for an hour, except that she performs ghusl." Some scholars are of the opinion that one should wait half a day or a full day, because it is customary for blood to flow and then stop. This is the view preferred by Ibn Qudamah and Ibn Uthaymeen, but the first opinion is more cautious. There is no harm in following a different opinion. If the blood returns during the menstrual period, it is considered menstruation. However, if it is yellowish or brownish discharge, it should be ignored.