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I pulled this one:
I can't find a reference to that anywhere, and the illustration of the Guild facuilty in The Assassins' Guild Diary doesn't suggest it. Female assassins just wear stylish black, just like male assassins.
(And, if I'm being pedantic, technically no female graduates of the Guild appear in the books. Female members, yes, but they're all self-taught; the first influx of female students, mentioned in the Diary and Night Watch, have probably graduated by the end of the series, but they don't get mentioned.)
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