Mishnah Negaim 4:3: There are [laws] by white hair that a healthy patch of skin does not have, and there are [laws] by a healthy patch of skin that white hair does not have. For white hair makes [the Nega] impure if it was in [the area of] a boil or burn, [if the hair was] concentrated or scattered, [if it was] surrounded [by the Nega] or not surrounded. There are [also unique laws] by a healthy patch of skin, for a healthy patch of skin makes a Karachat [Nega that balds the back of the head] or Gabachat [Nega that balds the top of the head], whether [the patch] was preexisting or not preexisting, and it prevents [purification of] someone who has turned completely white [by a Nega] and makes impure with any appearance, which is not true by white hair.

יֵשׁ בְּשֵׂעָר לָבָן מַה שֶּׁאֵין בְּמִחְיָה, וְיֵשׁ בְּמִחְיָה מַה שֶּׁאֵין בְּשֵׂעָר לָבָן. שֶׁשֵּׂעָר לָבָן מְטַמֵּא בַשְּׁחִין וּבַמִּכְוָה, בִּמְכֻנָּס וּבִמְפֻזָּר, בִּמְבֻצָּר וְשֶׁלֹּא בִמְבֻצָּר. יֵשׁ בְּמִחְיָה, שֶׁהַמִּחְיָה מְטַמָּא בַקָּרַחַת וּבַגַּבַּחַת, הֲפוּכָה וְשֶׁלֹּא הֲפוּכָה, וּמְעַכֶּבֶת אֶת הַהוֹפֵךְ כֻּלּוֹ לָבָן, וּמְטַמָּא בְכָל מַרְאֶה, מַה שֶּׁאֵין כֵּן בְּשֵׂעָר לָבָן:

Mishnah Negaim 4:4: Two hairs whose roots are black and tips are white are [still] pure. [Two hairs] whose roots are white and tips are black are impure. How much whiteness [is required]? Rabbi Meir says: Any amount. Rabbi Shimon says: [Enough] to be able to cut [them] with scissors. If [the hair] was one at the bottom but it split at the top and looks like two, it is [still] pure. A Baheret [type of diseased patch that creates impurity] in which there is white hair or black hair is impure, [because] we do not worry lest the area of the black hair reduced the [total area of the] Baheret [and made it smaller than the minimum size] because it does not have significance.

שְׁתֵּי שְׂעָרוֹת, עִקָּרָן מַשְׁחִיר וְרֹאשָׁן מַלְבִּין, טָהוֹר. עִקָּרָן מַלְבִּין וְרֹאשָׁן מַשְׁחִיר, טָמֵא. כַּמָּה יְהֵא בַלַּבְנוּנִית, רַבִּי מֵאִיר אוֹמֵר, כָּל שֶׁהוּא. רַבִּי שִׁמְעוֹן אוֹמֵר, כְּדֵי לִקְרֹץ בְּזוּג. הָיְתָה אַחַת מִלְּמַטָּה וְנֶחְלְקָה מִלְמַעְלָה וְהִיא נִרְאֵית כִּשְׁתַּיִם, טָהוֹר. בַּהֶרֶת וּבוֹ שֵׂעָר לָבָן אוֹ שֵׂעָר שָׁחוֹר, טָמֵא. אֵין חוֹשְׁשִׁין שֶׁמָּא מִעֵט מְקוֹם שֵׂעָר שָׁחוֹר אֶת הַבַּהֶרֶת, מִפְּנֵי שֶׁאֵין בּוֹ מַמָּשׁ: