Quick answer
Is brown discharge considered ḥayḍ?
Brown discharge during your established menstrual window is counted as ḥayḍ. The same discharge after the signs of purity is not ḥayḍ in the majority position, and prayer resumes.
During your recognized ḥayḍ window, brown and yellow discharge are counted as ḥayḍ across all four Sunni madhhabs. You do not pray, fast, or perform ṭawāf during this time.
After the signs of purity have appeared (the white discharge known as qaṣṣah bayḍāʾ, or complete dryness), the same brown or yellow discharge is not ḥayḍ in the majority view. This is based on the report of Umm ʿAṭiyyah (raḍiyallāhu ʿanhā): 'We used to consider yellow and brown discharge after the time of purity as nothing.' (Sahih al-Bukhari)
If brown discharge appears before any clear menstrual flow has begun, treat it cautiously: continue praying until clear flow establishes the cycle. Once flow is established, the Hanafi school may count the earlier brown discharge backward as part of ḥayḍ; the Shafiʿi and Hanbali schools count from when bleeding actually began.
Brown discharge that continues beyond the maximum days of ḥayḍ in your madhhab (10 days Hanafi, 15 days Shafiʿi/Hanbali/Maliki) is treated as istiḥāḍah, and prayer resumes with wuḍū before each prayer.
This summary draws on the established positions of the four Sunni madhhabs. For your specific situation, consult a qualified scholar.