What is the ruling on pink or brown discharge that appears 13 days after menstruation, knowing that it shows up when one is hungry or severely exhausted? Does it invalidate prayer and fasting?

If the pink discharge appears at a time when it cannot be menstruation—for example, if it is less than 13 days since the previous discharge stopped, or if the total duration of the discharge, including what preceded it, exceeds 15 days—then it is istihadha (non-menstrual bleeding). However, if it appears at a time when menstruation is possible, then it is menses (hayd). Pink discharge is a color of blood, and thus it is considered menses whenever possible. A brownish or yellowish discharge (kudrah) is not considered menses unless it is connected to the blood at its end. Brown discharge that is not connected to blood, as well as any discharge seen before the appearance of blood, is not considered menses. One must perform istinja’ (ritual cleaning) and wudu (ablution) for prayer until the usual red blood is seen.