Is the dark yellow fluid that comes in small amounts, followed by white discharge, considered a menstrual period, knowing that no blood is seen and the yellow fluid does not come daily?

Yellowish and brownish discharges are considered menstruation during the period of one's regular cycle, but not outside of it. The minimum duration of menstruation is one day and one night. Therefore, the yellowish discharge you mentioned is not considered menstruation unless it occurs during the period of your regular cycle and lasts for a full day and a night. If it is not within the period of your regular cycle, or if it lasts less than a day and a night, then it is considered istihadah (definition) (non-menstrual vaginal bleeding), and you are obligated to pray and fast, and you have the rulings of those who are pure.