What is the ruling concerning the day of menstruation when the blood flow stopped before completing seven days, but then turbidity (kudra) and yellowness (sufra) returned a day and a half after the usual end of the period, and is it obligatory to fast that day even if ghusl (ritual bath) was not completed before Maghrib?

If the yellowish discharge is likely to have occurred both before and after the end of your regular menstrual period, it is added to the closest time, which is after the end of your regular period. Therefore, your fast on that day is valid, because yellowish and brownish discharges after the regular menstrual period are not considered menstruation. You were not required to perform ghusl. If you missed the Dhuhr and Asr prayers thinking you were menstruating, you must make them up.