Our Gemara quotes a number of aggadic teachings in the name of Rava.
According to Rava, as a reward for Avraham’s statement (Bereshit 18:27) ve-anokhi afar va-efer – “Who am I to argue with God; I am but dust and ashes” – the Jewish people merited the mitzvot of sota (where the bitter water is made from dirt taken from the floor of the mishkan) and Para Aduma – the Red Heifer (which is made from ashes mixed with water).
In another statement, Rava taught that as a reward for Avraham’s statement (Bereshit 14:23) im mi-hut ve-ad serokh na’al – “I will take neither a thread nor a shoe strap” – the Jewish people merited the mitzvot of tefillin straps and the thread of tekhelet on their tzitzit.
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.