Shinuiy Marah-The law if the color of a Kesem/Maarah stain changed with time, and the need to bring the Shaala to the Rav as soon as possible:
Change of color from pure to impure: If while the Bedika or Kesem stain was wet it was a pure color and after it dried it became an impure color, then many Poskim rule that we judge the Bedika in accordance to its current dry color and she is thus impure, and so is the implied opinion of Admur. The Tzemach Tzedek, however, rules that if the color was actual white [as opposed to yellow] then it remains pure even if it afterwards changed to a tint of red. If, however, it went from a yellow or brown color to a red color then she is impure.
Change of color from impure to pure: If while the Bedika or Kesem stain was wet it was an impure color and after it dried it became a pure color, then it is invalid. If the color changed from a questionable color [i.e. brown] to a pure color, then some Poskim rule the Bedika is impure, while other Poskim rule that it is pure, and practically a Rav may be lenient when it is the woman [and not the Rav] who testifies that she thinks she saw a questionable color. Practically, due to the above, if the color changed from the time it was found to the time it was shown to the Rav, the Rav is to be told of this change if the change was significant.
When must the Shaala be brought to the Rav? Due to the above concept that a color can change with time and cause the Rav to give a mistaken ruling, one is to therefore bring the color to the Rav as soon as possible. In the interim one is to guard the Shaala in a clean and safe area. In the event there was a considerable time delay prior to showing the question to the Rav, then any relevant color change must be mentioned.
 See Tahara Kehalacha 6:5 and 9-12; Nitei Gavriel 117:3-6
 Admur 188:4 in name of Bach, Shach, Taz and Shvus Yaakov; Bach 188; Shach 188:3; Taz 188:1; Shevus Yaakov 2:74; Teshuros Shaiy; Tiferes Tzevi 6; Minchas Pitim 188; Maharash Or Zarua 87; Shiyurei Taharah 188:17; Shelas Yaavetz 1:12; Pischeiy Teshuvah 188:3; Poskim in Nitei Gavriel 117 footnote 4
Other Opinions: Some Poskim rule that if the color was originally pure then the sighting is Tahor as we say the air and cloth is what caused the color to change to look like blood although in truth it is not blood. This is not similar to a black spot in which we say “Dam Shalaka” that the red blood became tainted and turned black, as this can only apply from red to black and not from green or yellow to red. [Chacham Tzevi 46, brought in Admur ibid; Aruch Hashulchan 188:14; Teshuvah Meahva 364; Ikrei Daat 21:30; Sidrei Tahara 188:2; Chavas Daas 188:1; Mayim Chaim 36; Igros Moshe Y.D. 4:17; Poskim in Nitei Gavriel 117 footnote 4] Some Poskim conclude that due to this one may be lenient in a time of need. [Beis Meir; Nitei Gavriel ibid]
Opinion of Admur: Although Admur records both opinions, it seems that Admur ibid holds like the Bach, Shach and Taz from the fact he brought their opinion first and then concluded after bringing the opinion of the Chacham Tzevi that the Shvus Yaakov negates this opinion and rules like the Shach and Taz. However, see Tahara Kehalacha 6 footnote 14 who says Admur does not arbitrate in this case.
 Tzemach Tzedek Yoreh Deah 126; Meshivas Nefesh 2:30; Tahara Kehalacha 6:9 rules like this opinion; See Nitei Gavriel ibid
 See Chavas Daas 188:1; Divrei Chaim 2:81; Givas Shaul 63; Daas Kohen 188:2; Mahariy Asad 273; Maharsham 3:41 and 250; Shaareiy Tohar 63; Kinas Sofrim 30; Givas Shaul, brought in Aruch Hashulchan 188:18; Shoel Umeishiv Tinyana 1:33; Taharas Habyis p. 309; Tahara Kehalacha 6:12 and footnote 11; Nitei Gavriel 117:7-10
 Chavas Daas ibid and all Poskim ibid; Mahariy Asad that this applies according to all Poskim, even the Shach, Taz, Bach and certainly according to the Chacham Tzevi ibid; Taharah Kehalacha ibid footnote 19 that so is implied from Nidda 19a and 20b, and Maharam Merothenberg 378; See Nitei Gavriel ibid footnote 9
 Nitei Gavriel 117:10
 Taharas Yisrael 188; See Taharah Kehalacha 6 footnote 11
 Daas Kohen ibid; Aruch Hashulchan 188:18; Shaareiy Tohar 8:3; Igros Moshe Y.D. 4:17; Shiureiy Shevet Halevi p. 95; Taharas Habayis ibid; Poskim in Nitei Gavriel ibid footnote 10; See Taharah Kehalacha 6 footnote 11 and 19
 See Shiureiy Shevet Halevi ibid; Taharah Kehalacha ibid footnote 19; Nitei Gavriel 117:10
 See Taharah Kehalacha 6:5
 Such as if the color was really a shade of red and with time and exposure to air and sun changed to a brown color