How can the beginning and end of the menstrual cycle be determined, especially with intermittent yellow or brown discharge before the red blood, and continuous light yellow discharge after the cessation of blood? And is this discharge considered menstruation?

Yellowish and brownish discharges are considered menstruation if they occur during the regular period or are connected to blood. Therefore, do not perform ghusl until the bleeding, and any connected yellowish or brownish discharge, ceases, and you see purity signified by one of its two signs (dryness or the white discharge), provided that the menstrual period does not exceed fifteen days. If it exceeds this duration, then you are in a state of istihadah (non-menstrual bleeding). Your ghusl before ensuring purity is not valid, and your prayer and your husband's intercourse with you are not permissible at that time.