* The Pituchei Arbaat HaMinim (p. 266-7) discusses black dots that are caused by bug sprays that farmers use. He suggests two reasons to be lenient. First, he argues (based on the Terumat Hadeshen responsa 99) that since these black dots are part of the normal appearance of the etrog, they do not present a problem. Furthermore, the black dots are external to the etrog. Nonetheless, he concludes that one should be strict unless the dot can be removed. Similarly, Rav Dovid Miller (“Hilchot Arba Minim”) and Halachos of the Four Species (p. 22) say that a black dot on the upper part of the etrog invalidates it.
* Rabbi Zvi Sobolovsky ([http://www.yutorah.org/lectures/lecture.cfm/776494/Rabbi_Zvi_Sobolofsky/The_Laws_of_the_Daled_Minim The Laws of the Daled Minim]. min 4-5), however, explained that most of the etrogim we have do not have any issue with discoloration; the little black dots are just specks of dirt. Similarly, Rabbi Eliakim Koenigsberg ([http://www.yutorah.org/lectures/lecture.cfm/782108/Rabbi_Eliakim_Koenigsberg/A_Practical_Guide_to_Purchasing_Daled_Minim “A Practical Guide to Purchasing Daled Minim”] min. 32-3) quotes the Chaim Ubracha (p. 33 note 87), who says that black dots invalidate the etrog only if they develop because of rotting, which is not usually the case. Mishna Brurah (648:46) writes that a discoloration disqualifies an etrog only if it is noticeable upon a normal glance without staring at it carefully. </ref>
# An etrog that is completely green is invalid. If, however, if it started to become yellow, it is valid. <ref> The Mishna (34b) cites Rabbi Yehuda’s view that an etrog that is as green as grass is invalid. The Rosh (3:21) cites Tosfot’s assertion that an etrog that is green but will turn yellow over time is valid, since it must be a complete fruit in order for it to turn yellow. Shulchan Aruch (648:21) codifies this view. Mishna Brurah (648:65) writes that the Achronim decided that one should not rely on the fact that the etrog might potentially turn yellow later on unless it has begun to start doing so. Chazon Ovadia (p. 256) agrees. Rabbi Hershel Schachter (“The Halachos of the Daled Minim,” min. 33-5) cited the Mishkenot Yaakov’s opinion that the etrog is invalid even if it started to yellow.</ref>
# See note for a list of other ideal qualities. Besides for the qualities that Chazal specified, the niceness of an etrog includes its subjective beauty. <ref> Bumpy: Rama (Responsa 126) writes that the differences between a grafted etrog and a real etrog include: 1)A real one is bumpy, while a grafted one is smooth. 2)A real one has an indented oketz, while a grafted one has an oketz that protrudes. 3)A real one has a thick peel with very little juice, while a grafted one has a thin peel and a lot of juice. The Tiferet Yisrael (Mishnayot Sukka 3:6) says that a person ideally should look for an etrog that is very bumpy and has an indented oketz. Nitei Gavriel (p. 140) as well as [[Kashrut]] Arbaat Haminim (p. 8) codify this view.