What is the ruling on the fast of a woman who broke her fast for several days in Ramadan due to menstruation, then it became clear to her that what she thought was menstruation was not (a yellowish discharge), and does she owe expiation by feeding sixty poor people?

You are only required to make up for the days you missed fasting, and there is no expiation (kaffarah) upon you. The yellowish and brownish discharges connected to menstruation during the menstrual period are considered menstruation. So, if you broke your fast during those days, you did what was required of you. If you broke your fast outside your usual menstrual period due to ignorance of the ruling, then according to the majority of scholars and the Malikis, you are only required to make up the fast due to the presence of an interpretative understanding (ta'wil). If expiation (kaffarah) becomes obligatory, it must be given to sixty poor people. You are not required to fast an extra day beyond the days you missed as a precautionary measure; rather, you must perform ghusl (ritual purification), pray, and fast immediately upon seeing purity. If you do not see purity, then do not refrain from eating and drinking.