Is it valid to perform ghusl and prayer upon seeing the sign of purity (al-qassah al-bayda’ – white discharge) after the presence of yellow and red discharges, knowing that the purity appeared immediately after ghusl?

What a woman sees of yellowish or brownish discharge during her customary period is considered menses, and the sign of purity is complete dryness or seeing the white discharge. The questioner's prayer is invalid because she did not become pure until after ghusl (ritual bath), and she must make up all the prayers she performed before achieving proper purity.