הלכה על נדה 37:22

And that which they, may their memory be blessed, said (Niddah 19a) that there are five appearances of blood that the Torah made impure and the rest of the appearances of blood are pure - its content is [that it is] because the Sages knew in their wisdom that every blood that is not of those five appearances is not from an impure place. As there is a specific place in a woman, that only blood which comes from it is impure - it is called the source. And in the language of the Sages, it is called, "the room." And in their days, [since] they were experts in every wisdom, all the women were accustomed to practicing the law of the Torah for menstruation and discharge; such that a zavah would sit [out] for seven clean days, and a menstruant would immerse on the night of the eighth - and even if she was gushing all of the seven - since the blood stopped on the seventh while [it was] still day. And from when the troubles multiplied, the hearts (understanding) of sages and those that know the Torah shrunk and they did not want to rely on their opinions to judge appearances of blood at all. And therefore now at this time, anyone who has [a spot that has] an appearance that is at all red or an appearance of black - whether it is deep or light, meaning, that it is not deep - is impure. But green and white - even if it is thick - behold, it is pure. And a woman is believed to say, "I saw an appearance like this and I lost it." And if she said [it was] green or white, we render her pure (Niddah 20b).