To be considered ritually impure, blood has to come from the womb and be one of the colors below: red, black, red similar to a corner of crocus leaf, red like earth waters, and red similar to diluted wine.
What is red? – Similar to the blood of a wound. What is black? – Similar to black pigment used in creating ink. In truth, black is presumed to have been red initially, and then change colors, so it is not a new color. If we didn't say this, we would five colors, and not four. A “corner of a crocus” means the most brilliant, or the brightest, part of the crocus. “Earth waters” are obtained by taking earth from the valley of Beit Kerem, and floating water upon it. By diluted wine the rule means two times water and one part wine, as was the custom in the early times, when the wine produced was extremely strong.
How do we know that not every color of blood is ritually impure? Because in describing how a rebellious Sage disagrees with the Sanhedrin, the Torah uses the words, “If there be hidden from you a matter of judgment, between blood and blood.” They must have been arguing about the colors of blood from the womb.
How do we know to include exactly four shades of red? Because the Torah said, “her bloods” and then again “her bloods,” the first plural counting as two and the second also as two, to the total of four.
Art: Albert Anker - Still Life: Two Glass of Red Wine, a Bottle of Wine; a Corkscrew and a Plate of Biscuits on a Tray