Question for those who believe the Murex Trunculus is the chilazon of the Torah. What's the explanation for the fact that the dye it produces is purple, and you need to expose it to sunlight during the process to make it blue? Meaning, why do none of our traditional sources mention the fact that the only way to make techeiles with the chilazon would be to expose it to sunlight?

Does anyone have a suggested explanation? Or is it one of the things that remain a mystery, and isn't enough to counter all the evidence that it is indeed the chilazon of Chazal.

This question bothered the Chavos Yair so much that he concluded that techeiles must be purple, and not blue, as assumed by everyone. He was unwilling to deny his understanding that the chilazon is the Murex, so he had no choice but to change his paradigm for techeiles.

@robev I'd suggest posting this as a question on Mi Yodeya and simultaneously asking the experts at tekhelet.com/ask-ptil-tekhelet

@IsaacMoses there are so many questions here on techeiles and the Murex that I felt it probably overlapped a bunch of questions here that weren't answered (I did a search a little while ago). I might reconsider

@robev You could also consider improving / bountying an existing unanswered question.

Blue, purple, what's the difference?

Three options I see here:

1: it was known by the dyers who didn't record all the tips and tricks they used to get the best shades and so what?

2: whichever stuff came out blue got put in one pile and the purple stuff in another pile and no one really knew why. We use concentrated light and purified dye to fully debromimate it, but probably they never had it that perfect in either direction

3: purple is no closer to indigo than blue is. Or in other words, purple is Tekhelet as yellow is Yarok. Who said anyone cared what shade came out?

In any event you'll surely enjoy torahmusings.com/2017/06/tekhelet-color-perception-apprehension and the follow ups torahmusings.com/2017/06/tekhelet-perception torahmusings.com/2017/06/testament-true-tekhelet torahmusings.com/2017/07/tekhelet-radiant-sky

In the end the main argument for aiming for pure blue even if you think it used to be purpler is that even if they couldn't make it so perfectly blue back then, it's unlikely to be invalid since you're probably accepting the theories that there was a kosher range of hues. May as well aim in that direction.