Must I fast if I bathe after menstruation and then see brown or yellowish discharge, and how do I verify complete purification?

A woman determines the state of purity by one of two signs: (dryness or the white discharge). Any yellow or brownish discharge seen after purity is not considered menstruation. Therefore, the questioner's purity is valid, and she should have fasted that day. She does not need to repeat the ghusl (ritual bath); her initial ghusl is sufficient. She must make up for the days she missed fasting and the prayers she did not perform. We hope there is no sin upon her due to her lack of knowledge of the ruling.