Fatimah bint Muhammad (may Allah be pleased with her), who was brought up in the care of Amrah (may Allah be pleased with her), says that a woman of Quraysh sent Amrah (may Allah be pleased with her) a piece of cotton (I think he meant: with yellowness on it), asking her whether, when a woman sees nothing of the menstruation except this yellowness, she has become pure. She said: No, not until she sees pure white.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1592