Are sanitary pads sufficient to ascertain purity from menstruation, and what is the ruling on yellowish discharge accompanied by light bleeding after menstruation has stopped, and what is considered light yellowish moisture?

Purity is achieved by one of two signs: complete dryness when a piece of cotton is inserted into the place that becomes visible when a woman squats for her needs, or seeing the white discharge (al-qassah al-bayda'). Seeing discharges on the sanitary pad means that purity has not yet been achieved. If you verify your purity, then perform ghusl and pray. If you find any trace of blood, yellowish discharge (sufrah), or brownish discharge (kudrah), you are still menstruating. What you see of yellowish or brownish discharge after achieving purity and performing ghusl is not considered menstruation.