How can the time of ritual purity (Tuhr) from menstruation be determined if light yellow discharges continue to be present after menstruation, and complete dryness or a clear white discharge (qassah bayda’) has not occurred, knowing that these yellow discharges are natural for most days of the month?

If the yellow discharge mixed with menstrual blood exceeds the maximum duration of menstruation (fifteen days), you should refer back to differentiation:

If you can differentiate between menstrual blood and other discharges: What is not less than one day and one night and not more than fifteen days is considered menses, and anything else is irregular bleeding (istihadah). A woman experiencing istihadah fasts and prays, and she performs ablution for each prayer after its time has begun. If you cannot differentiate (because the discharge is uniform or a condition is not met): You refer back to your customary period. The days of your usual period are considered menses, and anything else is istihadah. If it is impossible to ascertain this: You observe the predominant length of menstruation (six or seven days), and anything beyond that is istihadah.

You must make up the prayers that you missed during the period of istihadah blood. Discharges invalidate ablution. If the discharge originates from the vagina, it is pure (tahir); if it comes from the urinary opening, it is impure (najis). If its discharge is continuous, its ruling is that of سلس (incontinence), so you perform ablution for each prayer after its time has begun and after taking precautions, and what comes out thereafter does not harm you.