Is the yellowish and brownish discharge after menstruation, which usually continues for more than 15 days, considered istihadah (prolonged vaginal bleeding outside of menstruation), knowing that the questioner performs ghusl after the fifteenth day, and she was previously asked about this and informed to wait for the duration of clear blood and consider the rest as istihadah?

Discharge (brown or yellowish) after the cessation of menstrual bleeding is not considered menstruation, based on the Almighty’s saying: {And they ask you about menstruation. Say, "It is discomfort"} [Al-Baqarah: 222], and Umm Atiyyah's statement: "We used not to consider any yellowish or brownish discharge after ritual purity as anything [i.e., as menstruation]."

So, if a woman sees ritual purity by one of two signs: dryness or white discharge, then ghusl (definition) becomes obligatory upon her. What she sees thereafter of brownish or yellowish discharge is not given any consideration, and she is not required to perform ghusl for them, unless the brownish or yellowish discharge occurs during the period of menstruation, in which case they are then considered menstruation.