Is ghusl required for previous menses that were not accounted for due to not recognizing the brownish discharge, and then ghusl again for the current menses?

Firstly: A distinction must be made between doubt (shakk) and certainty (yaqeen). Doubt is hesitation between two matters, while certainty is the firm conviction in something with 100% assurance.

Secondly: Yellowish discharge (sufrah) and brownish discharge (kudrah) connected to menstrual blood are considered menstruation. If you did not perform ghusl (definition) after their cessation, you are still considered menstruating. One ghusl, performed after the cessation of menstruation with the intention of رفع الحدث (removing the major ritual impurity), is sufficient for you.

Thirdly: If you see brownish discharge (kudrah) after seeing purity and dryness, it is not considered menstruation.

Fourthly: If you are certain that you did not perform the legislated purification after the cessation of menstruation, then you must make up all the prayers you performed in that state.

Fifthly: If you performed ghusl before being certain of purity and the cessation of menstruation, your ghusl is not legally valid, because one of the conditions for the validity of ghusl is the cessation of that which necessitates it.

Sixthly: If you perform ghusl after verifying purity by one of its two signs (dryness or the white discharge), then any yellowish or brownish discharge you see after that will not harm you.

Seventhly: Doubt about the presence of yellowish or brownish discharge, if it is merely an illusion or obsessive whispering (waswasah), should not be given any attention.