Cutheans were a people brought by the Assyrian emperor Shalmanessar from their native Cutha to Israel to populate the area of Israel left desolate by his exile of the Ten Tribes. Since then lived in Samaria, they are also called Samaritans. While in Israel, they were attacked by mountain lions, and they decided to convert to Judaism. They basically followed the written Torah the way they understood it, and in these areas they were even more meticulous than the Jews. However, they differed in matters of interpretation, which caused them to observe some laws incorrectly. Eventually, it was discovered that they still served idols, and they were declared to be non-Jewish, but prior to that they were a subject of long-standing disagreements.
The Sages gave all of the female Samaritans the status of a niddah , even while they are still very young, and all male the status of ritual impurity of a zav. They also considered all Cuthean males as though they cohabited with their wives while in the state of niddah. Why is all that?
Since the Cutheans did not accept the interpretation of the Torah by the Jewish Sages, they took no notice of the repetition “Woman... woman.” The Sages understood this to mean that even a young girl may become a nidah. Therefore, if it did happen, Jews would immerse the girl in the mikveh, while the Cutheans would not. The situation with the males was the same.
Furthermore, the Cutheans considered a woman a niddah when she had blood of any color, not just any of the four colors we learned above. They thought this to be an additional stringency. However, that really was a leniency: if their woman saw, for example, yellow color of blood, she would start counting her seven days – which was wrong. If she then saw red blood, it would be included in the seven-day count, which would thus finish earlier than it should. Thus, she would immerse while still a niddah, and the immersion would be infective.
Art: Pierre Auguste Renoir - Girl Gathering Flowers