Answer:
To determine the actual color of the discharge (before oxidation), a woman should place a small piece of cloth or toilet paper (kursuf) at the opening of the vagina. She should not insert it, but rather allow it to be held by the labial lips [Birgivi, Dhukhr al Muta’ahilin].
The female companions used to send these pieces of cloth to our Lady Aisha (may Allah be pleased with her) to determine the end of their menses, much the way you are inquiring now. She instructed them that a woman must not hasten to perform ghusl until she sees whiteness (Bukhari, Book of Menstruation).
“He laid upon you no hardship in the Religion” (Quran 22:78).
And Allah, Most High, knows best.
Checked and Approved by Shaykh Faraz Rabbani
