תכלת BLUE PURPLE — It is the blue dye of (obtained from) the blood of the Chilazon.
It is taught in a baraita that Rabbi Meir would say: What is different about tekhelet from all other types of colors such that it was chosen for the mitzva of ritual fringes? It is because tekhelet is similar in its color to the sea, and the sea is similar to the sky, and the sky is similar to the Throne of Glory, as it is stated: “And they saw the God of Israel; and there was under His feet the like of a paved work of sapphire stone, and the like of the very heaven for clearness” (Exodus 24:10), indicating that the sky is like a sapphire brickwork. And it is written: “The likeness of a throne, as the appearance of a sapphire stone” (Ezekiel 1:26).
נִתְכַּסָּה בְּטַלִּית: אֶלָּא מִצְוָה לְהָבִיא לָבָן וּתְכֵלֶת וְיַעֲשֶׂה, אֵימָתַי, כְּשֶׁהוּא תְּכֵלֶת, וְעַכְשָׁו אֵין לָנוּ אֶלָּא לָבָן שֶׁהַתְּכֵלֶת נִגְנַז.
"And make for yourselves tzitzit" (Numbers 15:38): the commandment is to bring white and techelet and make. When? When there is techelet, and now we do not have anything but white, since the techelet has been lost.