Are the 'Isha prayer and sexual intercourse considered valid if a woman performs ghusl after seeing a yellowish discharge on the fifth day of her menstruation, then her period returns before Fajr?

Yellowish or brownish discharge is considered menstruation if it occurs during the accustomed period or is continuous with blood. Ghusl (definition) (ritual bath) is not required until complete purity is observed, either by dryness or by the white discharge (al-Qassah al-Bayda’). As for the intercourse that occurred in the past while the brownish discharge was present, there is no sin or expiation for it, because you both believed you were pure. Sin and expiation are only for those who commit a prohibited act knowingly, intentionally, and by choice.