Reference for Niddah 37:44

כתיב (ויקרא יד) קיר

which indicates that blood is red.<span class="x" onmousemove="('comment',' As red is the usual colour of blood, all blood which has one of the five colours enumerated in our Mishnah (all of which are shades of red) is unclean. ');">³⁹</span>

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