Is it necessary to make up the prayer if a woman thought she was menstruating, but then it became clear to her that the blood was istiḥāḍah?

Yellow discharge may be menstruation if it occurs during menstruation or is connected to it before purification, in which case the rulings of menstruation apply to it. So, if the thin yellow blood discharge appears during your customary days, it is menstruation, and you are not required to make up the prayers for those three days. However, if the yellow discharge occurs outside of your customary menstrual period, it is not considered menstruation, and you must make up the prayers missed during those three days.