What is the factual difference between a Nega in the location of flesh and a Nega in the location of hair?
Rashi, Ramban #2 (citing the majority of commentaries) and Seforno: The sign of Tum'ah on the former is two white hairs, on the latter, two yellow hairs - after a patch of black hair has fallen out, 1 which explains why it is called 'Nesek'. 2
Ramban #1 (according to his interpretation of Rashi): The sign of Tum'ah on the latter is two yellow hairs that grow on black hair that is still intact. 3
Ramban #3: The latter, like the former, requires a patch of white skin 4 with one of the four appearances (Se'eis and its Toldah or Baheres and its Toldah), only instead of two white hairs, the Siman Tum'ah of the latter is two yellow hairs.
Ramban #4 (citing the Tosefta): The latter, like the former, requires skin without hair, and any appearance other than regular skin ('a white patch on black or a black patch on white') on which two yellow hairs grow.
What are the connotations of "u'Mar'eh ha'Nesek Amok min ha'Or"?
Ramban #1: It refers to the yellow (bright-colored) hairs that look deeper than the skin. 1
Ramban: It refers to the Nesek that turned white after the black hair fell out. 2
What is the meaning of "Nesek Hu"?