Are the light, reddish blood at the beginning of menstruation and the intermittent brownish blood at its end considered menstrual blood, and what is the ruling on prayer and fasting during them, especially since dryness is relied upon as a sign of purity?

If blood appears fifteen days or more after the previous menstruation, it is considered menstruation, and you must refrain from prayer, fasting, and everything that menstruation prohibits until the bleeding stops and you observe your usual sign of purity (complete dryness). If the total number of bleeding days exceeds fifteen days, it becomes Istihada (non-menstrual bleeding). However, if the total number of days is 14 days, all of it is considered menstruation.

If the bleeding stops and you observe purity, even for one day, perform ghusl (ritual bath) and pray. If the bleeding returns, you should stop praying. The most preponderant opinion is that the period of purity should not be less than a day, unless there is an indication otherwise, such as the cessation of bleeding at the end of her usual cycle.