Are the secretions with streaks of blood at the beginning of the period considered menstruation or purity? And how is the end of the period calculated with the continuation of different colors after the cessation of red blood?

As long as the blood comes to you for fifteen days and no more, then all of it is menstruation from the first blood that descended from you. You stop praying when it descends, and when it stops with dryness, you pray. If it returns, you stop again.