How can one determine whether blood that reappears before a full 15 days of purity is menstruation or istihadah, and what is the ruling on prayers performed during that period?

Summary

Blood that appears within 15 days and 15 nights and does not exceed that limit is ruled as menstruation, even if the habit or colour differs. If the bleeding continues past that limit, menstrual blood and istihadah are mixed and the woman is considered a mustahadah. For a mu'tadah mumayyizah, menstruation is determined by the strong coloured blood while the weak blood is istihadah. In this case the thin blood from 2 to 5 July is istihadah, so the prayers performed are valid, and the husband and wife incur no sin due to ignorance.

Written in our own words from the original fatwa — check the source before acting on it.