In the context of discussing forbidden loans, i.e. loans that charge interest, Rava asks the following question –
Why does the Torah juxtapose the Exodus from Egypt with:
1. Charging interest on loans (Vayikra 25:37-38)
2. Weights and measures (Vayikra 19:36-37).
3. Tzitzit – the fringes placed on four-cornered garments (Bamidbar 15:37-41)
Rava explains that God is teaching that just as He distinguished between the firstborn and ordinary Egyptian child at the time of the final plague, similarly He is able to –
1. recognize the individual who gives his money to a non-Jew in order to charge interest on a loan to Jews
2. know who buried his weights in salt in order to change their weight
3. recognize the difference between dye from the true-blue hilazon and kala ilan.
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 sky 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.
Kala ilan is apparently a plant – Indigofera tinctoria – a legume with red, pink or white flowers. True indigo dye was extracted from its leaves in Talmudic times and until recently. Today it has largely been replaced with synthetic dyes. It would appear that this plant dye was very similar in its appearance to the tekhelet of the hilazon, but was a much cheaper alternative. Only through complicated examinations could the two dyes be told apart. The rishonim explain that Rava in the Gemara is not talking about someone who mistakenly wore tzitzit with kala ilan, but someone who offers the dye for sale, claiming that it is the real tekhelet.