Niddah 20 - Shades of Color
If a woman's blood coming from her womb is " red like that of a wound ," it makes her ritually impure. Rav Yehudah explained, "Like the blood of a slaughtered ox." Ami the Beautiful said, "Like a wound of a little finger that was cut, then healed, and then cut again, and not the little finger of any person, but only a man who has not wed a wife, until the age of twenty." Rav Nachman said, "Like the blood of bloodletting."
Ameimar, Mar Zutra, and Rav Ashi all went for bloodletting. When they extracted the first tube of blood of Ameimar, he said, "That blood color is just like in the rule (above) about a woman." On the second tube he mentioned, "The color has changed." Rav Ashi after this remarked, "I, who cannot tell the difference between the two shades, should never rule on it, even for such an important matter as to permit a woman to her husband."
Ifra Hurmiz, the mother of the Persian king Shapur, observed the laws of niddah. She once sent her blood to Rava, who said, "It is the blood of desire, and it is not like other bloods, but it is pure." (Some say, it is impure, and Rava was demonstrating his deep knowledge.) She said to her son, "Look how wise Jews are!" He said, "Maybe he just stumbled upon the answer." She then sent Rava sixty types of blood, and he identified them all, except for the last one, the blood of a louse. He had help from Above though, since together with his answers he sent her a fancy comb used to kill lice. She exclaimed, "Jews, you dwell in the chambers of the heart!"
Art: Jacob Jordaens - An old man with a raised finger