Quotation for Berakhot 18:11
<big><strong>גמ׳</strong></big> מאי בין תכלת ללבן אילימא בין גבבא דעמרא חיורא לגבבא דעמרא דתכלתא הא בליליא נמי מידע ידעי אלא בין תכלת שבה ללבן שבה.
GEMARA What is to be understood by "between blue and white"? Is it to be supposed between a lump of white wool and a lump of blue wool ? That can be distinguished also at night ! But it means, between the blueness in it and the whiteness in it.
Shulchan Arukh, Orach Chayim
From when does one bless on the ציצית [that he puts on] in the morning? From the time when [there's enough light that] one can tell the difference between the techeilet (blue) in it and the white in it. And if he puts it on starting at dawn, some say he makes a blessing, and this is our practice (Mordechi - chapter 2 of Tractate Megillah). And if he put it on earlier than that, such as during [the time of year when we recite] selichot, he should not make a blessing over it [right away], and [then] when it becomes light he should touch [the ציצית] and makes the blessing (Tashbe"tz).
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