Is it permissible to act upon the opinion of Ibn ʿUthaymeen that brownish and yellowish discharges are not considered menstruation at all, despite not being reassured by this opinion and fearing the consequence of punishment?

The opinion that yellowish and brownish discharges are not menstruation at all is the view of Ibn Hazm and the chosen opinion of Ibn Taymiyyah. One may follow whoever holds this view. If the blood stops and the yellowish or brownish discharge is connected to it, then it is menstruation. But if the blood ceases and the woman sees purity, she should not pay attention to any yellowish or brownish discharge that appears thereafter, unless it occurs during her usual period.