Was my first and last ghusl valid, knowing that I doubted my purity after seeing dryness, then saw a brownish discharge, then performed ghusl again upon seeing dryness, and what should I do in this situation?

You must ignore intrusive thoughts (waswās) and pay no attention to them. Purity is achieved by one of two signs: dryness or the white discharge. Whichever of these the woman sees first, she should perform ghusl (ritual bath) and pray. Performing ghusl upon seeing dryness is valid. As for any yellowish discharge that reappears thereafter: if it is after the usual period of menstruation, it is not considered menses. If it is within the usual period of menstruation, then it is menses, and performing ghusl from it is valid.