What is the ruling regarding a woman who became pure after Asr, by dryness, then a simple yellow spot appeared at the time of Ishāʾ; is she considered pure or is she still in the menstrual period?

Yellowish or brownish discharge is considered menstruation if it occurs during the usual period. It is not considered menstruation after the usual period has ended, unless it is continuous with blood. Therefore, if yellowish discharge appears after your usual period has ended and you have purified, it is not considered menstruation. However, if it appears during your usual period, it is menstruation, and you must perform ghusl (definition) (ritual purification bath) after it ceases. If you did not do so, you must repeat the prayers that were performed without ghusl.