The Torah commands us in Bamidbar 15:38 to attach tzitziyot on the four corners of our garments, and insert a thread of blue in each tzitzit.
Speak to the children of Israel and you shall say to them that they shall make for themselves fringes on the corners of their garments, throughout their generations, and they shall affix a thread of sky blue [wool] on the fringe of each corner.
(Translation via Chabad.org)
The Gemara (Menachot 42b) informs us, however, that the blue thread cannot be dyed with just anything. We must dye the thread with the blood of a certain sea-creature called the chilazon.
Why must we dye the thread using the blood of the chilazon? Why does the Gemara raise such a big stink about people using kala ilan, indigo, to dye the threads instead of the blood of the chilazon? The color was exactly the same.