We have learned (see daf, or page 38) that although the Torah mentions the color tekhelet on many occasions, it is not really a shade of color; rather it is the dye from which this color is made. It is clear, however, that the blue dye of thetekhelet was taken from a living creature called a hilazon.
There were similar color dyes available in the time of the Mishnah – so similar, in fact that it was difficult to distinguish between the true tekhelet and the counterfeit tekhelet. The Gemara refers to a dye called kala ilan, which means “dark blue,” or, perhaps, the word kala itself means “dark blue” and the word ilan – “tree” in Hebrew – was added in order to indicate that it was dye that was produced from a vegetative source rather than from a living creature.
It appears that the kala ilan referred to in the Gemara is True indigo, or indigofera tinctora L. True indigo is a shrub one to two meters high. It has red, pink or violet flowers. It may be an annual, biennial, or perennial, depending on the climate in which it is grown. The plant is a legume, so it is rotated into fields to improve the soil in the same way that other legume crops such as alfalfa and beans are.
The blue dye is obtained from the processing of the plant’s leaves. This plant was the single most important source of blue dye for woven fabrics, and it grew mainly in India, although it also could be found in other areas including the Middle East. Only in recent years has synthetic indigo overtaken this traditional method of producing blue dye.
It appears that although kala ilan and tekhelet were very similar in appearance, kala ilan was much cheaper to produce than the tekhelet, and it was almost impossible to distinguish between them.