What is the ruling on the fast of one who fasts with the intention of making up for a fast as a precaution, while a brownish discharge occurs and she performs ghusl from it and prays?

If the brownish discharge (kudra) is connected to blood, it is considered menses, and fasting and prayer are not valid during it. One must make up for the missed fasts and perform ghusl (ritual bath) after seeing purity. However, if the brownish discharge is not connected to blood, meaning it appears after seeing purity and performing ghusl, then your fasting and prayer are valid, and there is no need to make up for missed fasts or repeat ghusl.