Is expiation (kaffarah) obligatory for one whose husband had intercourse with her after her menstrual bleeding clearly ceased, but before the yellowish and brownish discharges ceased, and is the expiation for each day or a single expiation?

It is clear from your question that you deliberately transgressed by having intercourse with your husband during a time you believed to be menstruation. What is incumbent upon you is sincere repentance. Nothing is incumbent upon you other than repentance according to the majority of scholars. As for the Hanbalis, yellowish discharge connected to blood is considered menstruation by some, and therefore, an expiation is incumbent upon you, which is one dinar or half a dinar. It is not incumbent upon your husband due to his ignorance. If the act is repeated, the madhhab according to the Hanbalis is that only one expiation is obligatory. Paying the expiation is more cautious.