It is a mitzva from the Torah to dye one (or according to some opinions, two) of the strings of the tzitzit a special sky-blue color. This color is reminiscent of the blue sky, which is in turn reminiscent of God in heaven.

The sky-blue color was produced from the secretion of a certain sea-snail. The Talmud relates that this snail would emerge from the sea every seventy years (Menahot 44a). At a certain point, the snail stopped appearing on land and the tradition with regard to its identification was lost. Alternatively, the identity of the sea-snail may have been lost due to historical, political, or economic factors.

The use of sky-blue dye for the strands of the tzitzit ceased back in the period of the geonim, probably around 1300 years ago. In the absence of the sky-blue dye, it has been customary to wear tzitzit with just white strings, which is still a fulfillment of the mitzva. Starting from the nineteenth century, several attempts were made to reconstruct the original sky-blue color, while at the same time, a polemic arose over the correct identification of the snail and the extent to which we can renew the sky-blue dye of tzitzit nowadays. A leading voice for the opinion that the correct sea creature had been identified, and that the use of the sky-blue dye should be renewed, was the Radziner Rebbe, Rabbi Gershon Henoch Leiner, who claimed that it was produced from the cuttlefish. He was opposed by many of the halakhic authorities, who claimed that this dye should not be used, as they were not convinced it was produced from the correct source. Additionally, some argued that it is impossible to recreate this tradition nowadays, and that dyeing some of the strings contradicts the custom to wear only white fringes (whereas by strict halakha, the strings that are not sky-blue may be any color).

Later, Rabbi Y.I. Herzog, first Ashkenazic Chief Rabbi of the State of Israel, identified a different sea creature as the source of the sky-blue dye mentioned in the Torah. On the basis of his work and that of scholars who followed him, some people today wear sky-blue fringes produced from the murex trunculus. There are still some people who wear sky-blue fringes dyed in the manner prescribed by the Radziner Rebbe. The majority of the public still wears tzitzit with just white strings.