Gilberre posted this as a comment. I'm not sure how many will see it that way so I'm re-posting it here.
"Therigiua is now called Essouerra and is in Morroco The Nice lady doing the Dying workshop at Viceroy knows that the imperial roman robes dted thryian purple were made of silk and was able to define the actual dye chemical and the acid animal fibre dying process I will have to use so now I am a lot further along in using my dye.
I will have to look up Byantium as another way to research seashell dyes.
What colours were they getting ?
The shells fron Assouerra give either a stunning Purple (Male) or vibrant blue (female)
must find out how they are sexed for my documentation !"
Thanks for the great info Jenny & Gilberre!