Are the secretions that come down after the menstrual period, which contain threads of blood, considered menses requiring ghusl, or are they istihadah that does not prevent prayer?

If a woman sees dryness (purity), she should perform ghusl and pray. Any yellowness she sees afterward should not be considered anything unless it is connected to blood, in which case it is menstruation. If she sees purity and performs ghusl, then the blood returns, she reverts to being a menstruating woman, and she must perform ghusl after it ceases. The fundamental principle is that any blood a woman sees is considered menstruation unless it occurs at a time when it cannot be considered menstruation. Therefore, the blood she saw on the seventh day and thereafter is considered menstruation, and she must perform ghusl after it ceases.