Since only blood and not other uterine emissions render the woman niddah, it is important to consult a rabbi if any off-color hence questionable emission is found*
1. The impurity of a stain or actual blood found on a cloth depends on its color: any reddish color is impure, whether it is very light or very dark. Black is also considered impure. 2. Any emission of colors such as white or even beige like bee's wax, green, yellow, gold or blue are all pure. Even if the emissions are thick and smooth like blood they are still considered pure.
3. Concerning the colors of emissions it is important to consult a rabbi who has expertise in determining the status of colored emissions. This applies whether it is to determine the status of a stain or a checking for a Hefsek betahara, or for the seven clean days. At times the expert rabbi will determine a stain pure because of its color, which the layman would consider impure. In short, when in doubt about a color ask the expert rabbi.
4. A brownish emission is considered impure if found on the checking cloth for a Hefsek Betahara, or of a checking after feeling her uterus open. However, for subsequent checks during the seven clean days or for stains, brown is considered pure. The rabbi should be consulted whenever a brownish colored emission or stain is found as it is often reddish but might not be. Rabbinic advice should be sought until a person feels comfortable with deciding the status of colors.
* The laws of this chapter are adapted from Taharat Habayit Vol. 1 Chapter 6.