Is the brownish discharge that appears after purification from menstruation, and continues for a day or two, considered to invalidate fasting and prayer, especially if it is interrupted by dryness then reappears?

A woman recognizes purity by one of two signs (dryness or white discharge). She must perform ghusl immediately. Any brownish discharge she sees after purity is not considered menstruation. However, if she sees clear blood after seeing the sign of purity, during a time when menstruation is possible, then she has returned to being a menstruating woman, and she must perform ghusl after the bleeding stops. Purity that occurs intermittently during menstruation is valid purity. When in doubt about the occurrence of something, it is attributed to the nearest time it could have occurred. Your fast is valid if you saw purity then brownish discharge, and ghusl is not obligatory upon you unless you are certain you saw blood.