The information in this article comes from a video entitled True Blue the story of Tekhelet part III at https://www.tekhelet.com/video-library-2/
According to Dr. Baruch Sterman, former Israeli Chief Rabbi Herzog, who wrote a doctoral thesis on Tekhelet was not aware how to produce blue dyed woolen strings from the Murex Trunculus mollusc (also known as Hexaplex Trunculus). The experts had informed Rabbi Herzog, that the Murex Trunculus mollusc only produces the dye for purple but not blue.
Years after the death of Rabbi Herzog, Professor Otto Elsner, by accident discovered that at a critical stage in the dye process, if the dye material of the Murex Trunculus was exposed to strong sunlight, the dye would produce blue colored wool instead of purple.
Dr. Baruch Sterman elaborated that the Murex Trunculus mollusc actually contains a mixture of 3 dye molecules. Dibromoindigo (a dye molecule containing 2 bromine atoms), Monobromoindigo (a dye molecule containing one bromine atom), and Indigo.
During the Leuco stage of dyeing, if the dye is not exposed to strong UV radiation from the sun, the dominant dye molecules will be the molecules that contain Bromine and thus the end result of the dye process will produce purple.
However, if the dye is exposed to strong UV radiation from the sun during the Leuco stage of dyeing, the radiation will cause most but not all of the bromine atoms to separate from the dye molecules, transforming the dye molecules into indigo.
Plant indigo contains no bromine atoms at all. Thus if we find ancient blue dyed fabric, we can now determine the source of the dye. Namely, if it has trace elements of Bromine Dye molecules, it came from a Chilazon (Murex Trunculus).