In all cases that the sight of blood deems the woman impure one must verify whether or not a found color is Halachically defined as blood. Certain colors are automatically deemed as blood colors while others are questionable. In all cases of a questionable color, one must bring the color to an expert Rav to analyze and determine whether she is pure or not. One may not rule on his own whether a certain color is pure until he has personally learned how to do so from a Rav expert in these matters. It is likewise forbidden for one to be stringent and impurify a questionable color in order to avoid bringing the question to a Rav. The most prominent colors held in question which must be shown to an expert Rav are colors that can be viewed to have a reddish tint. In all cases, the color must be shown to the Rav even if only a minute area of the color is questionable, while the remainder is of a Kosher color.
The following are the shades of color and their Halachic status:
- Pure colors:
- Bright Yellow [like of an egg yolk or Esrog]
- Impure colors:
- Black. All shades of black are impure colors. This includes colors that have a black tint, or shade, even if it is a very light color.
- Red. Includes colors that have a red tint or shade even if it is a very light color.
- Questionable-Ask Rav:
- Brown: The color brown, which is similar to the color of the coffee bean, is a dispute in Poskim as to its status. Some Poskim rule brown is an impure color. Other Poskim rule brown is a pure color.
- gray with blackish tint,
- dark yellow
 Shulchan Aruch Chapter 188:1; Taharah Kehalacha 6; Kitzur 1:6
 Taharah Kehalacha 6:2
 Taharah Kehalacha 6:3
 Taharah Kehalacha 6 Supplements 1; See there regarding the Hefsek Taharah that some are stringent by bright yellow, although most are lenient and so is the practical custom.
 Admur 188:2; Taharah Kehalacha 6:1; See Nitei Gavriel 116:10
Other opinions: Some Poskim rule that a shade which tints to black but is not black is pure. [Sidrei Taharah 166]
 Taharah Kehalacha 6:1
 See Taharah Kehalacha 6:2; Nitei Gavriel 116:1-5
 Lechem Vesimla 188:2; Salmos Binyamin 10; Beis Shlomo 2:4; Teshuras Shaiy 1:136
 Sidrei Tahara 188:1; Yaavetz 1:44; Chochmas Adam 111:1; Taharas Yisrael 188:2