If this brownish discharge that the woman sees comes before she sees that she is pure (i.e., seeing a white discharge which indicates that the period is over), then it is part of the period. If it comes after she sees that she is pure, then it is not part of the period and does not affect her praying and fasting.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) says in his essay al-Dima’ al-Tabee’iyyah li’l-Nisa’ (p. 19) concerning discharges that may interrupt the period:
The third kind is yellowish or brownish discharge, where a woman sees yellowish blood like blood from a wound, or brownish, somewhere between yellow and black. If this happens during the period or following on from it, before the woman becomes pure, then it is part of the period and comes under the rulings on menstruation. If it comes after she becomes pure, then it is not menstruation, because Umm ‘Atiyyah (may Allaah be pleased with her) said: “We used not to regard the brownish or yellowish discharge that comes after a woman becomes pure as being anything that mattered.” Narrated by Abu Dawood with a saheeh isnaad.
He also said in Fataawa Arkaan al-Islam (p. 258):
The basic principle is that if a woman becomes pure and her period is over for certain – which means that she sees the white discharge that is known to women – then any brownish or yellowish discharge or spotting or wetness that comes after she has become pure is not menstruation, so it does not prevent her from praying or fasting, and it does not prevent a man from having intercourse with his wife, because it is not menstruation.
But she should not rush until she has seen that she is pure, because some women, when the bleeding lessens, rush and do ghusl before seeing that the period is over (the white discharge mentioned above). Hence the women of the Sahaabah used to send the cotton with yellowish discharge to the mother of the Believers ‘Aa’ishah (may Allaah be pleased with her), and she would tell them: “Do not hasten until you see the white discharge.”.