Must I fast the days on which blood came down after the turbid discharge and yellowish discharge, and repeat the prayers I missed?

If the questioner means "changed" in terms of number, not time, then what she saw of brown discharge and blood on the fifth day is considered menses, which prohibits prayer and fasting, and she does not become pure until she sees the white discharge (al-qassah al-bayda') or complete dryness (al-jufūf). It does not seem that it changed, as it appeared on the fifth day, and she mentioned that her period is five or six days.

However, if she means change in time, then if fifteen days have passed since her last menses, and she sees blood that is fit to be considered menses (remains for a day and a night), then it is menses according to the majority of scholars. The more correct view is that it is so, because the regular period may come earlier or later.

Yellow or brownish discharge (ṣufra or kadira) that is connected to blood or that occurs during the period of menses is considered menses. However, if it appears after purification and outside the period of menses, it is not considered menses. Whoever fasted while menstruating must make up for those days.