How can the end of menstruation be determined, and how can slight yellowish discharge be distinguished from doubt about purity? What is the necessary period to ensure purity? And is it necessary to repeat past prayers if the time of purity was determined incorrectly?

Purity is defined by one of two signs: dryness or the white discharge. The criterion for dryness is inserting a piece of cotton into the place and it coming out without any trace of blood, yellowish discharge, or brownish discharge. The usual moistures are not taken into account. If a woman ascertains the presence of a yellowish discharge, even if slight, she is still considered to be menstruating if it is within the period of her usual cycle or connected to blood. Mere doubt or delusions are not to be heeded. If a woman sees purity and performs ghusl, then sees a yellowish discharge afterward, she should not heed it unless it is within the period of her usual cycle or connected to blood. If a woman is certain that she missed prayers due to incorrect calculation of purity, then it is more cautious to make them up. As for mere doubt, it is not to be heeded, for the default is the validity of worship and the exoneration of one's responsibility.