Why does it say "Lechu"?
Radak: It does not refer to going, rather, to be zealous. The same applies to "Lechu v'Nachsheva Al Yirmeyah" (Yirmeyah 18:18), "Lechah Na Anasecha v'Simchah" (Koheles 2:1) and similar verses.
What will be the dispute?
Rashi: Hashem and you will dispute, to see who did improperly to the other. If you did improperly, He will still give to you hope (opportunity) to repent.
Radak: The litigant tells his claim against his opponent, like "Sham Yashar Nochach Imo" (Iyov 22:7), "v'Im Yisrael Esvachach" (Michah 6:2). Hashem will tell the goods that He did for You, and the evils that you did in front of Him. You, since you cannot claim against Him, will regret your evil deeds.
Malbim: Why do you not repent, like I instructed you?
Why does it say "Yomar Hashem"?
Rashi #1: He always tells you so, e.g. "Al Pi Hashem Yachanu" (Bamidbar 9:20).
Rashi #2: Above (17), Hashem said "Limdu Heitev
What are "ka'Shanim" and "ka'Tola"?
Rashi: "Ka'Shanim" is stained in front of Me like red of Shanim. "Ka'Tola" is like a dye used to dye red. They are seeds, and there are worms in each one.
Radak: Shanim is red. Tola is Tola'as Shani, a crimson dye; it is redder. Sin is compared to red, a strong color that is seen from afar. A stain in a garment of this color does not depart via laundering - "Im Techabesi ba'Neser v'Sarbi Lach Boris Nichtam Avonech Lefanai" (Yirmeyah 2:22). Teshuvah removes the stain.
Metzudas Tziyon: Shanim are threads dyed red, like Tola'as Shani.
Malbim: Tola is the worm whose blood is used to make the dye Shani. Tola has two severities. (a) Its color is intrinsic; Shani received its color via the worm. Even if you are so rooted in sin that it becomes intrinsic in you (refer to 1:18:4:4), i.e. one who sins due to denial and rebellion, [Teshuvah will cleanse it]. (b) Tola dyes others; Shani does not. Even if you sinned and made the Rabim sin - this is why Ya'adimu is Hif'il 1 - the sins will not become white like snow, but they will be [white] like wool.
What is the grammatical form of "Yalbinu" and "Ya'adimu"?
Radak: They are Po'el Omed (the action applies only to the subject 1 ).
Even though they are Hif'il, they do not act on others. (PF)
Which color of wool will they resemble?
Radak: They will be like white, clean wool; the adjective is omitted. We find like this - "ha'Nosen Sheleg ka'Tzemer" (Tehilim 147:16). The noun is mentioned, but not the adjective. The same applies to "Zera Anashim", "Yamusu Anashim" (Shmuel I, 1:11, 2:33). My father said, if the sins are ka'Shanim, which is not so red, I will atone for them, and they will be white like snow. If the sins will be very red ka'Tola, they will be like wool, which is not so white.
Why does it discuss the change of color?
Malbim: You need not fear to repent, for even if your sins are red (severe), they will whiten, if "Rachatzu Hizaku" (16). You will be purified from all your sins.
