What is the reliable sign of purification, and is it necessary to be completely certain of the whiteness of the discharge when experiencing scrupulosity?

A woman recognizes purity by one of two signs: complete dryness, where no trace of blood, yellowish discharge (ṣufrah), or brownish discharge (kudrah) remains; or seeing the white discharge (al-qaṣṣah al-bayḍā’). If a woman sees white discharge, it indicates her purity, and she must perform ghusl (ritual bath) and pray. Any subsequent yellowish or brownish discharge is not considered menstruation unless it is connected to blood. One must not pay attention to obsessive thoughts but rather hasten to perform ghusl as soon as a woman sees a sign of purity. Furthermore, the maximum duration of menstruation is fifteen days.