Niddah, Chapter 3, Mishnah 1. This mishnah continues the discussion begun in the mishnah before it, which taught which shades of discharge render a woman temei'ah. There, five colors were listed. Here the Mishnah returns to that list and identifies each color precisely, so that a person examining a stain would know exactly what shade is meant.
"Eizehu adom?" What is the red that was listed? "Kedam hamakah": like the blood of a wound at the moment it first begins to flow, a fresh, bright red. That hue is the red which is forbidden.
"Shachor", black, is defined as "kacheres": like the black dye used by shoemakers, a substance familiar to everyone in the time of the Mishnah. That depth of black is the standard. The Mishnah adds a rule of measurement: "amok mikan, tamei", if the shade is darker than that black, it is likewise tamei, "dehe mikan, tahor", but if it is fainter and lighter than that black, it is tahor.
The third color on the list was "kekeren karkom", like the brightest point of the crocus. The Mishnah explains: "kabarur shebo", the reference is to the most brilliant portion of the crocus plant, its clearest, most vivid reddish part. That is the shade intended, and not the duller portions of the plant.
The fourth color was "meimei adamah", earthy water, water that pools upon the ground and takes on its tinge. Which earth? The Mishnah specifies the earth "mi'Bikas Beis Kerem", from the valley of Beis Kerem, a well known location in Eretz Yisroel in those days, "u'meitzif mayim", and one floats water over it. The brownish red water standing above that particular soil is the fourth of the forbidden shades.
The fifth color was "hamazug", diluted wine, a reddish purple tone. The Mishnah gives its recipe: "shnei chalakim mayim v'echad yayin", two parts water mixed with one part wine, and specifically "min hayayin haSharoni", from the wine of the Sharon region, whose deep purplish color was known. Wine of that type diluted in that proportion produces the shade the earlier mishnah had in mind.
These, then, are the five colors that carry tumah, because a discharge of such a shade is presumed to be blood. A discharge of any other color is not presumed to be blood at all, and it does not render her temei'ah.