Gemara

(Mishnah - Akavya ben Mahalal'el): Yarok blood is Metamei (makes a woman a Nidah);

Chachamim are Metaher.

Nidah 19a (Mishnah): There are five Tamei colors of blood -- red, black, like the radiance of saffron, like water on dirt, and like mixed wine;

Akavya is Metamei Yarok; Chachamim are Metaher.

R. Meir says, (even) if a Yarok Kesem (stain) is not Tamei, the blood is a liquid.

R. Yosi says, it is neither.

19b - Question: Doesn't Akavya also learn from "Dameha... Dameha" that there are only four Tamei colors?! (Black was red, but it was stricken.)

Answers: He agrees. (He learns another color from elsewhere). Alternatively, Yarok is a stricken form of one of the four colors.

Question: Chachamim are Metaher. This is like the first Tana!

Answer: The first Tana considers Yarok Safek Tamei. Chachamim consider it definitely Tahor.

Question: R. Yosi says that it is neither (Tamei or Machshir). This is like the first Tana!

Answer: The first Tana is R. Yosi. One who says a Halachah in the name of the one who taught it brings redemption to the world.

Chulin 47b: From R. Noson, we are Machshir if the lungs are Yarok;

(Beraisa - R. Noson): A women lost two sons through circumcision. Her third baby was Yarok. I counseled her to postpone the Bris until the blood was absorbed. Her baby survived.

(Rav Sama brei d'Rava): If the lungs are like hops, saffron or an egg, it is Tereifah.

Question: If so, what is the case of Yarok that is Kosher?

Answer: If they are Yarok like leeks, it is Kosher.

Eduyos (Mishnah 5:6): When Akavya ben Mahalal'el was dying, he told his son 'retract from the four things about which I argued with Chachamim (including Yarok blood). I held like what I heard from the majority, but you heard my opinion only from me. Follow the majority that you heard, who argue with me.'

Rishonim

Rambam (Hilchos Isurei Bi'ah 5:6): Only blood that flows from the Cheder is Tamei. Therefore, if a white or Yarok liquid flowed, even if its consistency is like blood, since its appearance is unlike blood, it is Tahor.

Rosh (Nidah 2:4): The Ri learns from here that Stam Yarok is the color resembling an egg yolk, or gold, which leans to red. Also Chulin 47b connotes that Stam Yarok is like hops, saffron or an egg. Also Sukah 34b mentions Yarok k'Karsi, which implies that Stam Yarok is not like leeks. Bereishis Rabah expounds "va'Yarak Es Chanichav" - Horikan (he made them Yarok) with gold. Also "its wings are Yerakrak gold" (Tehilim 68:14) shows that Yarok resembles red and gold. Nowadays, no one is proficient in the appearances to rely on his Chachmah to distinguish between the different bloods. Even in the days of the Gemara, some Chachamim refrained from seeing blood (to rule on it). All the more so nowadays one may not be Metaher any blood leaning to red, unless it is white or Yarok like gold. Even though the first Tana holds that it (Yarok) is Safek Tamei, the Halachah follows Chachamim.

Ran (Shevuos 3a DH Yachol): The first Tana said that there are (only) five Tamei colors of blood (and Yarok was not among them). This is why the Gemara asked that Chachamim say like the first Tana. The Ramban says that even though the Halachah follows the first Tana regarding other colors, for the Stam Mishnah is like him, regarding Yarok the Halachah follows R. Yosi, for he is called Chachamim in our Mishnah. Also, this opinion is called Chachamim in Eduyos (5:6), and we always rely on what it says in Eduyos. Also, Akavya himself retracted at the end of his life. Presumably, he retracted to hold like R. Yosi, who is totally Metaher, and not like the first Tana who was in Safek, for the Mishnah in Eduyos brings only the opinions of Akavya and R. Yosi. Nidah 21a is like R. Yosi. It is Metamei only the four colors, and everything else is Tahor. Also the Rambam rules like R. Yosi.

Ran (DH u'Mar'is): Even though nowadays we are not proficient in the appearances and we are Metamei anything reddish, we are not concerned for Yarok or white. Chachamim decreed only about Dam (perhaps this should say 'Adom (red)' - PF) and its derivatives. These are not included, for the Halachah follows R. Yosi regarding Yarok.

Me'iri (Nidah 19a DH Omar): The Tana'im argue about green. Yellow is like saffron, which is Tamei! Even though green is not red at all, perhaps Akavya holds that it was red, but it was stricken.

NOTE: The Gemara's second answer was that Yarok blood was red, but it was stricken. How can the Me'iri explain the first answer?

Magihah (6): I did not find another Rishon who holds like the Me'iri. The Me'iri supports the Shlah (brought in Pischei Teshuvah below).

Poskim

Shulchan Aruch (YD 188:1): Every appearance of blood is Tamei, whether it is very faint or deep. The same applies to a black appearance. The only Tahor appearances are white and Yarok, even like wax or gold, and all the more so Yarok like leeks or grass.

Beis Yosef (DH Devar): The Rosh, Rashba, Ran and Rambam are totally Metaher Yarok. They brought proofs.

Beis Yosef (DH Kosvu ha'Tosfos): Tosfos and the Rosh say that Yarok is like an Esrog, but not like leeks, which do not resemble red. They hold that there was no Hava Amina to be Metamei green blood, which is not all reddish. The Hava Amina is only for Yarok like gold. Akavya holds that it has a leaning to red, and Chachamim disagree. We are Metaher even Yarok that leans to gold or wax. The same applies to anything without any redness. Yarok was mentioned only because one opinion holds that when it is like gold, it resembles red.

Bach (DH v'Ishah): If a woman saw a Yarok stain, and afterwards the outside became red, letter of the law she is Tamei, like we say about black. When the blood leaves the body, it is stricken and blackens, like blood of a wound. Similarly, we are concerned lest when blood left the body it became Yarok, and when it dries it returns to its initially red color. Perhaps Akavya and Chachamim argue only about when it was red the entire time, but if the outside became red when it dried, even Chachamim are Metamei. Therefore, a Rav should not permit a wet Yarok or white stain until it dries.

Shach (3): The custom is unlike the Bach. We do not suspect problems without grounds (that it will return to a red appearance).

Pischei Teshuvah (2): The Shlah says in the name of Maharshal that Yarok requires investigation, for Mahari Mintz said in a Teshuvah that Gedolim forbid it. Therefore, one should not be quick to permit if it is like wax, and surely not if it is like gold, unless there are other reasons to say that it is not Dam Nidah. If she found Yarok blood with Hargashah (her entire body shook, she felt the uterus open, or she felt a liquid flowing), she should be stringent. If she found a stain without Hargashah, we are lenient. If it was found when cleaning the area, there are reasons to be lenient or stringent. Sidrei Taharah says that the Acharonim omitted this, for they disagree, but a Ba'al Nefesh (a spiritual person) should be stringent when appropriate. The Chasam Sofer (149) ruled that if a Chacham permitted a reddish appearance on a Bedikah cloth, her husband may be stringent, for this is normal for Prushim (very pious people). Even if he did not conduct Prishus at the time of the Nisu'in, it is common for people to become Prushim later. Even if she screams in protest, he was never obligated (to fulfill Onah when there is room for a Parush to be stringent).

Rema: Also the color called 'blue' is included in Yarok.