It's not easy being green
The common practice today is to use esrogim that have begun to turn yellow, even if they are still mostly green. This is not universally accepted; some, such as the Brisker rabbanim, insist on fully yellow esrogim.
In the third chapter of Maseches Succah, the Mishnah discusses the status of a green-colored esrog. The Gemara there rules in favour of Rabbi Yehuda, who says “If it is green like a leek [hayarok kikarsi] – passul”. The Gemara initially assumes that Rabbi Yehuda is of the opinion that greenness is an absence of hidur, and contrary to the Torah commandment that the fruit be “pri eitz hadar”. However, the Gemara concludes that this cannot be Rabi Yehuda’s reasoning as we find elsewhere that Rabbi Yehuda, unlike his colleagues, permitted the use of a dried-out esrog, based on an interpretation that the word hadar does not require that the esrog be ‘beautiful’, but rather that it be ‘mature’.
Based on the Gemara, Tosafos conclude that if one has an esrog that is currently green but will turn yellow over time, it would be kosher even according to Rabbi Yehuda. This leniency is also adopted by the Rosh, the Tur, and the Shulchan Aruch. The Taz and Magen Avraham explain that use of such an esrog is permitted even before it turns yellow, so long as one knows that it will definitely do so.
However, the Bach argues with the Tosafos’s interpretation of the Gemara and rules instead that a green esrog is pasul for two reasons: it is not hadar, and it may be immature. This stringency was also recommended by the Bikkurei Yaakov, Beis Meir, Shiyarei Knesses Hagedolah, Olas Shabbos, and Mishkenos Yaakov. Several of the aforementioned authorities also argue that even Tosafos would only allow the use of a green esrog after it has turned yellow and is actually mature.
The Rishonim and Acharonim argue whether an esrog that is currently green but will turn yellow over time is kosher for use. There are more lenient and more stringent opinions, but the Mishnah Berurah rules based on a ruling of Maharil mentioned in Magen Avraham, that one should only use a green esrog if it has already begun to turn yellow.
Several Acharonim quote the Zohar who not only considers green esrogim kosher, but actually believes that a green esrog is the preferable, stating “just as Esther was yerakrokes.” The Gemara in Maseches Megillah (13A) says that “Esther was yerakrokes, yet a thread of grace was strung upon her.” In other words, Esther had a green complexion, or olive-toned skin, but Hashem miraculously caused to find favor in the eyes of everybody who saw her. How the Zohar connects a green queen with the kashrus of an esrog is not clear to most Acharonim, however the Bikkurei Yaakov hints that they share some secret connection but does not elaborate on this secret.
In conclusion, all that can be said with clarity is that when it comes to an esrog, “it’s not easy being green”.
Image: A green Queen