Is the brown discharge that precedes menstruation by one day, interspersed with purity or yellowness, accompanied by pain, considered menstruation that necessitates abstaining from worship, or istihadah that necessitates prayer?

There is a difference of opinion among scholars regarding yellowish (ṣufra) and brownish (kudra) discharges not preceded by menstrual blood. Some do not consider them menstruation unless preceded by blood (Shaykh Ibn Uthaymeen), while others consider them menstruation as long as they occur within the possible timeframe (the majority of scholars).

It is permissible to follow an opinion in this matter:

According to the opinion of the majority (brownish discharge is menstruation): What you did by refraining from prayer upon seeing the brownish discharge during your customary period is correct. If it is followed by a continuous yellowish discharge, that is also menstruation. If you see purity after the brownish discharge during your customary period, perform ghusl and pray. If the brownish discharge or blood returns during your customary period, you revert to being in a state of menstruation, provided that the total duration of bleeding and purity does not exceed fifteen days. According to the preferred view of Shaykh Ibn Uthaymeen (brownish discharge is not menstruation): It is obligatory for you to pray until you see menstrual blood. You must also perform istinja' (washing) from this yellowish and brownish discharge because it is impure.