1) [RE: Insights, 19:1]
Yellow colored stains:
The Beis Yosef in183 brings the shitos that chardal is the colour all though he disagrees in pshat he seems to agree in din
See also the Aruch Hashulchan 185:8. A doctor in shiur told me that included in the discharge is some thing with a latin name that translates as the yellow matter it is made up of the unfurtilised egg and the hormones related to it. So both descriptions are really related to the same thing.
Brown see the Aruch Hashulcha sham 9 that says not to be machmir at all.
2) [RE: Insights 20:2]
The Ohr Hayashar quotes a Nitziv in SHiltos Vayara 13 7 that Yalsa was marriesd a few times and that this story was probably during the previuos marriage.
3) [RE: Insights to 20:3]
The Tos in AZ is talkjing about a mistake in sevorah not in secondary issues.
The Divrei Chaim writes that in Maros it is not considered a psak even according to the shitos that the issue is the Kavod of the first as the issue is in eye sight and not in Psak, I dont have aDivrei Chaim to check up how he says it and it has been some time since I saw it I think that the Beer MOshe Debrezyn also says some thing Like that and maybe quotes cholkim.
(Later...) I got hold of a Divrei Chaim and in chelek 2 siman83 he differentiates between maros in the time of chazal and zman hazeh that then it would be a psak and today it is just anything red and even a lady can pasken.
This chilik is interesting as the Gemorah before told of the deaf lady whose friends would show her their kesomim.
Keep the papers coming
1) Regarding yellow colored stains- that is simply a quote of Rabbeinu Yonah, who says just that.
Brown is not just one color. There are so many different shades of red-brown that it is impossible to pass a single ruling on all of them. Rav Gustman, Zatzal, would break up the Mar'eh to individual dots (pixels?), and if there was a section in the brown that was actually red he would be Metamei.
2) Beautiful answer!
3) That's correct. Our Tosfos in their first explanation, and Tosfos in Chulin, suggest that the second Chacham can only correct a mistake caused by bad eyesight etc.