Etz Yosef on Bamidbar Rabba

Tanchuma Shlach Siman 29:

[(Numb. 15:38:) THAT THEY MAKE <TZITZIS> FOR THEMSELVES.] When you make them, they shall not be from <something already> made. Thus one must not extract threads from the cloak <itself> and make <tzitzi> from them. Rather <it is> a command to get <fresh> white and blue threads to make them. When <did this rule apply>? When there was <a real> blue, but now we only have white, because the blue has been hidden. (Ibid., cont.:) ON THE CORNERS OF <THEIR GARMENTS>: Not in the middle, but on the corner. (Ibid., cont.:) A TWISTED THREAD (petil). Thus it is necessary to twist (petol) them. R. Meir said: Why does blue differ from all <other> kinds of colors? Because blue resembles [the sea, the sea resembles] the firmament, and the firmament resembles the throne of glory. Thus it is stated (in Exod. 24:10): AND THEY SAW THE GOD OF ISRAEL, AND UNDER HIS FEET THERE WAS SOMETHING LIKE BRICKWORK OF SAPPHIRE, LIKE THE HEAVENS THEMSELVES FOR BRIGHTNESS.

Etz Yosef

“Ptil” teaches that the strands must be twisted together—that is, they must be plied, and each individual thread should not remain separate by itself. Nevertheless, after the fact, if he did not twist them together, they are not invalidated on that account. (N.Tz. and E.R.)

“That techeiles resembles, etc.” Techeiles is wool dyed with the blood of the chilazon, a sea creature whose appearance resembles the appearance of the sea. Its blood is black like ink, and it is found in the salt sea.

The wool is first soaked in lime and then washed until it is clean. The blood and the wool are placed into a boiling vat together with dyeing ingredients such as ohel and tzarif, in the manner practiced by dyers, so that the wool will absorb the color. (Rabbeinu Bachya, Parashas Shelach.)

The Mizrachi wrote that techeiles is wool dyed a yarok color with the chilazon. That is, the chilazon is a sea creature that rises from the sea once every seventy years, and the appearance of its blood is yarok, resembling the sea.

Here, yarok is best understood in its older rabbinic sense as a blue-green or sea-colored hue, rather than necessarily the modern Hebrew meaning of plain green.