What is meant by "the duration of the custom" according to jurists concerning yellowness, and is only the duration of the blood counted, or the blood and yellowness together?

The Hanbalis are of the view that yellow and brownish discharges during the menstrual period are considered menses. As for a woman experiencing her first period (al-mubtadi'ah), any blood exceeding one day and one night is considered doubtful until it recurs three times. Some scholars believe that a habit (of menstruation) is established with a single occurrence. Some scholars are of the view that yellow and brownish discharges are not menses unless they are connected to blood. So, as long as the woman has not seen purity, the yellow and brownish discharges are menses. But if she has seen purity and then experiences yellow or brownish discharge, she should not pay attention to it. It is permissible to follow a trustworthy scholar and adopt the easier opinion.