Explanation: [In Shir ha'Shirim, the man praises his beloved's voice,] but she does not praise his sweet voice, rather, that he is white (or clear) and red.

Question: Even without this, [we decreed that] they are Nidos from birth! (Maharsha - we are thinking that this is lest Yisre'elim mingle with them.)

Answer: That [was not due to mingling, rather, due to Safek Tum'ah. It] was only for minors, for [Kusim] do not expound "v'Ishah" [to include a minor], like it says in the Gemara (32a);

However, adults, who separate [due to Dam Nidah] and immerse, they are Teme'os only because they are Yoshev (conduct like a Nidah) due to every [color of] blood. (Sometimes this causes them to immerse too early, which is not Metaher.)

Explanation: The Gemara explains that Tachton (a mattress under) a Bo'el Nidah is like k'Elyon (something above; Tosfos will explain this) a Zav. It is Metamei only food and drink.

Explanation #1 (Rashbam): Elyono Shel Zav is when he pushed down one pan of a balance scale, and thereby lifted food and drink in the other pan.

Rebuttal: In Shabbos (83a), we call this Heset (moving), when a Zav pushed down a pan of a scale;

We learn Heset from "Asher Yiga Bahem ha'Zav", and we learn Elyono Shel Zav in our Sugya from a different verse!

Explanation #2: Elyono Shel Zav is a garment that is partially on a Zav, and partially on the ground;

This is not Heset. That is only when he moves all of it, like it says "what is touching that is like [touching] all of it? This is Heset."