What is the ruling on sexual intercourse after the cessation of menstruation, followed by the appearance of yellowish discharge, then the wife is surprised by the reappearance of menstrual blood several hours after intercourse? And is expiation required of her?

Yellowish discharge that appears after the cessation of blood is considered menses if it occurs within the customary period, and the same applies to blood that returns. If intercourse takes place during the intervening period of purity, there is no sin in it. However, if it occurs during the yellowish discharge, while believing it to be menses, then both spouses must repent and seek forgiveness, and some scholars hold that they must also offer expiation.