Does a drop of blood or brown discharge that appears after ghusl for menstruation invalidate purification and fasting?

If the brownish discharge you saw was not blood, then it is not considered menstruation unless it was connected to blood or occurred during your habitual period. However, if it was menstrual blood, then it is menstruation, and ghusl (ritual bath) is obligatory after its cessation. Therefore, you must make up the prayers you missed from Dhuhr until the dawn of the following day, as they are a debt upon you.