Looking and ruling on a Shaalah of a stain during nighttime:[1]

Initially-Lechtachila: In general, one is to only look and rule on the state of [a questionable[2]] Marah-stain during the day, using sunlight [until sunset[3]], and not at night using a candle [or artificial light].[4] [Thus, in general, a Rav may avoid ruling on a questionable color during nighttime.]

Bedieved and time of need: Nonetheless, Bedieved, if one ruled on the state of a stain during nighttime opposite candlelight [or artificial light such as an electric lamp], it is valid.[5] [Likewise, in a time of need, such as she needs to go to Mikveh that night and the like[6], it is customary for Rabbanim to initially rule on a question even at nighttime, opposite an electric lamp.[7] This especially applies regarding a Kesem stain which is only Rabbinical.[8] Furthermore, some Rabbonim are even initially accustomed to rule on a question at night even if it is not a time of need.[9] However, even when checking at night one should not use florescent light as it is known to change the appearance of the color to be brighter than it really is.[10] Likewise, in the event that when looking at the color at night it looks merely questionable, the Rav should avoid ruling upon it at night and rather wait until morning to check it opposite sunlight when it may indeed appear to be pure.[11]]

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[1] See Taharah Kehalacha 6:16; Nitei Gavriel 119:12-17

[2] However, if it is clearly white or light yellow, then it is not even a question and does not need to be ruled upon during the day, and one can look at it at night and discard it.

[3] Divrei Yatziv 68; Nitei Gavriel ibid

[4] See Rama 196 “The check should be during daylight and not through candlelight.”; Levush 196:4; Toras Hashelamim 188:1; Sidrei Taharah 188:1 and 19; 190:93; Chochmas Adam 117:17; Lechem Vesimla 188:5; Aruch Hashulchan 196:15 and 44; Daas Torah 38:3; 188:1; Pades Rimonim 196 Sifsei Daas 2; Birkeiy Yosef 188; Ben Ish Chaiy Tzav 2:12; Sefer Haeshkol 44; Meiri Nidda 17

[5] Rama ibid; Meiri ibid; Toras Hashelamim ibid; Nitei Gavriel ibid footnote 20 in name of many Poskim including Lechem Vesimla ibid, Beis Shlomo Y.D. 2:41, Pardes Rimonim ibid, Yad Sofer 29, Shevet Halevi 4:106

[6] See Nitei Gavriel 119:16

[7] Taharah Kehalacha ibid footnote 24 that so he received from Rav Wozner and Rav Fruend that they would rule even at night; Nitei Gavriel 119:12

[8] Sidrei Taharah 188:1; 190:93; Daas Torah ibid

[9] See Nitei Gavriel ibid footnote 20 in name of many Poskim including Lechem Vesimla ibid, Toras Hashelamim ibid, Beis Shlomo Y.D. 2:41, Pardes Rimonim ibid, Yad Sofer 29, Shevet Halevi 4:106

[10] Taharah Kehalacha ibid; Shiureiy Shevet Halevi p. 96; Nitei Gavriel 119:15

[11] Taharah Kehalacha ibid; Nitei Gavriel ibid