What is the ruling on pink or light red blood, or blood mixed with white discharge or with red streaks, that appears after seeing the white discharge and performing ghusl from menstruation? Is it considered menstruation, necessitating re-performance of ghusl and invalidating prayer and fasting?

All the aforementioned scenarios are considered menstruation. When you see purity by one of its two signs, you must promptly perform ghusl (ritual bath). Then, if the blood returns, you are considered to be menstruating again, whether the blood is black or red, little or much. You must perform ghusl every time the blood stops, and you become menstruating again every time the blood returns, as long as it is possible for it to be menstruation.