The fourth perek (=chapter) of Masechet Menahot, Perek ha-Tekhelet, which begins on today’s daf (=page) continues discussing a number of loosely related commandments that have two or more parts. Unlike the previous perek, which dealt with commandments whose different parts depend on one another, (e.g. the four parshiyot in tefillin or the fourtzitziyot on a four-cornered garment) this perek focuses on mitzvot that do not depend on each other, so that each part is viewed as a separate commandment that can be fulfilled in-and-of itself (e.g. the tefillin of the arm and the tefillin of the head, or the tekhelet of tzitzit and the tzitzit themselves).
The Torah mentions the color tekhelet on many occasions, but it is not really a shade of color; rather it is the dye from which this color is made. Various discussions in the Gemara make it clear that the blue dye of the tekhelet was taken from a living creature called a hilazon. Because of the many Gemarot that describe the hilazon, it is difficult to identify one particular animal that meets all of the criteria, and there are many different opinions regarding its classification. The consensus of most opinions is that the hilazon is the snail “Murex trunculus” that is found on the shores of the Mediterranean Sea in the north of Israel. This creature has a unique liquid dye (that is not the animal’s blood), which, when mixed with other materials, produces the blue tekhelet color described in the Torah. Already during Talmudic times the use of tekhelet became a rarity, and within a short time its true source was forgotten.
It appears that the color of the tekhelet dye was a dark blue containing shades of green, which is why the sources compare it both to the sea and to grass.