Is white underwear in the clean days minhag or halacha?

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The Talmud (Shabbat 13b) refers to the shivah neki’im as “white days” (yemei libun). Shulchan Aruch Yoreh Deah 196:3 mandates wearing clean undergarments during the shivah neki’im. Rema there writes that the custom is to wear whites, but that in a pressing situation, a clean undergarment will do. Gra comments there that this was hinted at in the Talmud. Torat HaShlamim there notes that a colored undergarment that would affect analysis of the color of a stain would not be acceptable, as a matter of basic halacha.

In practice, white undergarments during the clean days are widely considered a binding custom, with clean undergarments acceptable in a pressing situation (or after the fact). That is to say, whites are not an absolute halachic requirement, but have become the halachic standard in practice.

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