In June 2006, I critiqued another figurative midrash essay, also based on a midrash on megillat Esther, which stated that Esther had green skin. I pointed out that the theme/figurative meaning the author developed as a possible interpretation of the midrash, namely that this was hidden Divine guidance, is in fact something that is explicitly mentioned in the text of the gemara, rather than something that needs to be tentatively suggested because of difficulties in believing that Achashverosh was from Mars and Esther was from Venus. Once again, looking at the original source and its context could have helped a lot. I pointed out as well, there, that is was likely that the midrash did not intend that Esther had green skin like a Venusian but rather that she had a sallow complexion, which while not pretty is perfectly normal. Indeed, just as the gemara derives Esther's complexion from hadas (willow, and from hadasa, her name), the term sallow derives from the myrtle (arava).
Thinking both midrashim over, we might be able to suggest a joint theme which prompted each midrash.
Firstly, while I emphasized context regarding Vashti's tail, it is important to note that while the gemara was an original context, it was not the only original context. The gemara weaves in the two midrashim about Vashti developing leprosy and about the angel Gabriel giving Vashti a tail into a larger midrashic tapestry of the king commanding that Vashti appear before him and his guests in the nude, and how Vashti was a prutza and given other circumstances would have wished to display herself in this manner.
However, as the gemara makes clear, these are citations of other sources to answer this difficulty. Megillah 12b:
אמר להם אחשורוש כלי שאני משתמש בו אינו לא מדיי ולא פרסי אלא כשדיי רצונכם לראותה אמרו לו אין ובלבד שתהא ערומה שבמדה שאדם מודד בה מודדין לו מלמד שהיתה ושתי הרשעה מביאה בנות ישראל ומפשיטן ערומות ועושה בהן מלאכה בשבת היינו דכתיב אחר הדברים האלה כשוך חמת המלך אחשורוש זכר את ושתי ואת אשר עשתה ואת אשר נגזר עליה כשם שעשתה כך נגזר עליה ותמאן המלכה ושתי מכדי פריצתא הואי דאמר מר שניהן לדבר עבירה נתכוונו מ"ט לא אתאי א"ר יוסי בר חנינא מלמד שפרחה בה צרעת במתניתא תנא [בא גבריאל ועשה לה זנב
Thus, the (setama di) gemara asks why she would have refused to appear, given that she was a prutza. It answers based on two earlier sources. Thus, it cites Rabbi Yossi bar Chanina that something (some derviation, presumably from nigzar aleha as mentioned by Tosafot and a Yerushalmi) teaches that she developed leprosy.
(Alternatively, "this difficulty" of why she did not come teaches that she developed leprosy. But its reads better as a derivation from the text, because that seems to be the usual semantic role of melamed.)
And it offers another answer, from an earlier source, a brayta, that the angel Gavriel came and fashioned for her a tail.
Alas, we cannot examine the original context of these statements, and just have as evidence the way they are used in the gemara. Perhaps it was identical, perhaps not. We see how the gemara reads these midrashim. But, for example, the nigzar aleha pasuk which might be the source for the leprosy occurs in the megillah after the punishment, as the first verse in the second perek. Could it be that the derasha did not even intend to offer a reason for her not coming, but rather was simply describing her middah kineged middah punishment, and/or giving a reason that the king did not take her back? Thus, caution may be appropriate here. Even if it was a reason for her not to appear (which is how the gemara understands it), was the original context one that she was expected to appear in the nude? That is also not clear, and there is no way of resolving it.
However, I did notice a theme common to both Esther's "green" skin and the set of midrashim about Vashti developing leprosy or sprouting a tail. That is, the focus on divine intervention directing events behind the scenes as opposed to natural occurrence of events as a result of Vashti and Esther's beauty.
On the plain level of the text, Vashti's beauty is part of her downfall, for it prompts the king to appear before him and his guests to show off, and she refuses. Similarly, Esther's ascendancy to the throne is due to her exceptional beauty. Thus, וַתְּהִי אֶסְתֵּר נֹשֵׂאת חֵן, בְּעֵינֵי כָּל-רֹאֶיהָ.
Yet, there is a clear undercurrent in the text that all this is the hidden direction of God. It is evident in the coincidences that all fit together. It is evident most explicitly in Esther's call to have the Jews fast, such that her appearing before the king does not get her killed, as well as in Zeresh's pronouncement that Haman is fated to fall before the Jews. Chazal pick up on this theme and elaborate on it. This is what they do in the midrashim about Esther and Vashti.
God is cast as granter and remover of beauty, and it is that beauty and lack thereof that influences events. Thus, Vashti, the text notes, is exceptionally beautiful. Assume she was not to appear in the nude, but simply to impress everyone with her exceptional beauty. We still might understand why she could refuse, but let us assume that a beauty would not mind impressing everyone. (Or, if she was to appear nude, assume that she was a prutza, an exhibitionist. It is all the same.) Yet God is directing events behind the scenes, and He wishes for her to refuse to come before Achashverosh and his guests. God lowers the mighty and lifts the humble, and also grants and removes beauty at will. This is what this pair of midrashim is saying. Leprosy, especially in Tanach (or in midrashim), is often viewed as Divinely placed. Thus, Pharaoh gets leprosy for having Sarah in his palace. Naaman gets leprosy, and wants it removed by Elisha. Generally, leprosy has a ritual performed by a priest to remove it. Thus, it is no accident that just now she develops leprosy. Similarly, if she sprouted a tail, she did not develop this naturally. The brayta states that the angel Gavriel came and fashioned for her a tail. Thus, God, through his emissary, removes her beauty.
The opposite is the case for Esther. According to her midrash, she really was not so pretty, having a sallow complexion. Yet Hashem extended to her a chut shel chesed so that she appeared beautiful. Perhaps besides the derasha on her original name, Hadassa, the aforementioned phrase, וַתְּהִי אֶסְתֵּר נֹשֵׂאת חֵן בְּעֵינֵי כָּל-רֹאֶיהָ, implies that she was not intrinsically beautiful but rather just everyone who saw saw her chain. (Perhaps this is then the source of the shidduch description of someone as "full of chain.") Thus, this is God granting her beauty in order to advance the narrative and get her into the palace.