Chullin 89: The Colour Tekheleta

The Gemara continues to discuss statements made by Avraham.  Rava teaches us that in the merit of Avraham who says, "I will not take a thread nor a shoe-strap" (Bereishit 14:23) his descendants received two commandments: the three of techelet, tzitzit, and the strap of the tefillin. 

Rava goes on to ask why techelet, a colour, is specified from all of the other colours for this mitzvah.  He responds that techelet is similar to the colour of the sea, which is like the sky, which is like a sapphire, which is similar to the throne of Glory.  The proof forgot his is from Sehmot (24:10) and Yechezkel 1:26. 

The rabbis argue about the colour of techelet:

• rabbis say that it is the colour of grass or leeks
• Rambam and Rav Se'adya Gaon say that it is the colour of the clear sky
• Rashi says that it is a green or turquoise colour
• Rabbi moshe HaDarshan says that it is the colour of the sky near evening when there is a purplish shade to the blue

To determine the colour, some people searched out the colour of a chilazon, marine creature, which is the source of techelet.  False techelet is known as kala ilan, which is an indigo colour.