Is it permissible to exercise ijtihad (independent reasoning) in estimating the number of menstrual days if yellowish discharge was not considered menstruation for two years due to ignorance, and how can her correct habitual period be ascertained, knowing that she observes purity at the end of the sixth or seventh day?

Yellowish and brownish discharges are considered menstruation if they occur during the regular period, or if they are continuous with blood. In such cases, one should neither pray nor fast while they are present. There is another opinion, however, which does not consider them menstruation at all; this is the view chosen by Ibn Taymiyyah. It is permissible to adopt the less preferred opinion after the act has occurred and its rectification would be difficult.