In your Background to Shabbos 89b I saw:
"ISTIS - (O.F. wesde) woad, a plant that produces a dye colored indigo (RASHI (68a and 79a), RAMBAM, BARTENURA, TIFERES YISRAEL); (however, Rashi on our Daf, Pesachim 56b, Bava Kama 101b and Nidah 50a translates it as "crog", safflower, which produces a red dye).
I was curious where the source is for this quote from the Rambam. I see that in Kil'ayim 2:5 the Rambam in Perush ha'Mishnah translates Istis as "Wesdes; Al Nil'ag" - do you know how to translate those words?
In Hilchos Shabbos 9:14 the Rambam writes, regarding making a dye essence, "If someone puts Istis into Mei Karkom, making it all green ('Yarok' - which can also be translated as yellow)..." This seems to say that Istis makes a yellow or green dye.
Pesach, Yerushalayim
The Radbaz (Teshuvah 2:685) writes that the dye referred to as "Nil" in Arabic is the Istis of the Gemara and its color is similar to Techeles. The Me'iri in Makos 5a describes the color the same way and calls it in Arabic "Rilag" (which should be corrected to say "Nilag").
In Hilchos Shabbos, the Rambam is simply stating that by adding Istis to yellow Karkom, one can make a green colored dye-essence.
Mordecai Kornfeld