What is the ruling on yellowish and brownish discharge after seeing the clear discharge, and is it considered menstruation during the accustomed period or within 15 days, and how can a woman determine her accustomed period in this situation?

If a menstruating woman sees purity (cessation of bleeding), then any yellowish or brownish discharge that appears afterwards is not considered anything, even if it occurs within her usual period. This is based on the statement of Umm Atiyyah (may Allah be pleased with her): "We used not to consider the brownish and yellowish discharge after purity as anything." Therefore, what you relied upon by not paying attention to the yellowish and brownish discharge after your purity is correct.