What will be the law if two yellow or red hairs grow in the Nesek?
Rashi: Based on the word "v'Se'ar", it is Tahor.
What do we learn from "Tahor Hu v'Tiharo ha'Kohen"?
Rashi: If the Kohen declares Tahor a person who is Tamei, he is not Tahor. 1
See Sifsei Chachamim.
What is the law if only one of these was fulfilled (the Nesek did not grow, and black hair did not grow in it, or it grew, and black hair grew in it)?
Bechor Shor, Hadar Zekenim (5): According to R. Yonason, who says (Bava Metzi'a 94b) that the prefix 'Vov' implies even one (unless it says 'together'), he is Tahor. According to R. Yoshiyah, who says that Vov implies both of them, he is Tamei.
What do we learn from "v'Im b'Einav Amad"? It already says that if it did not spread, he is Tahor!
Da'as Zekenim: Perhaps this is to teach the law of black hairs in the middle (they are Metaher).
QUESTIONS ON RASHI
Rashi writes that "v'Se'ar" teaches that also two yellow or red hairs in the Nesek are a Siman Taharah. What is extra to be expounded?
Perhaps Rashi explains like R. Yonason (refer to 13:37:151:1); we expound the prefix Vov, for the Torah could have written 'Oh Se'ar.' (PF)