Are the thick, viscous yellow secretions mixed with white – which continue all day after 12 days from menstruation and were not preceded by dryness – considered menstruation, or not, and is ghusl (ritual bath) obligatory in this case?

The yellowish discharge connected to menstruation is considered menses if it occurs during a time suitable for menstruation. So, if it ceases and dryness is observed before the completion of fifteen days, all of that is considered menses. However, if the duration of the blood and the yellowish discharge connected to it exceeds fifteen days, then the woman is considered to have irregular bleeding (istihada).