What is the ruling on fasting the two days during which menstrual blood did not flow, given the presence of brownish discharge on the first day, and should the Dhuhr prayer of the first day be repeated, and is this discharge considered the beginning of menstruation?

This yellowish or brownish discharge that you saw before seeing menstrual blood is not considered menstruation, because yellowish or brownish discharge is not considered menstruation except if it occurs during the habitual period or is connected to blood. As long as these discharges occur outside of the habitual period, they are not menstruation, and your fast is valid, and you do not need to make up any of the fasts or prayers.