Parashath Noah, Laws of Ziziyoth, Page 14:
1. The formula for the dye, as also the process through which the threads were dyed, was lost when the centres of Jewish settlement moved away from the Mediterranean. Although some scholars attempted to rediscover the process of making and using the dye, their findings have not been accepted by the majority of halachic authorities. Thus, in view of the severity of the prohibition of sha'atnez, one may not rely on the dyed threads available today to render a lenient ruling.
(Note from the poster: This note relates to item 2 which talks about linen tzitziot - they can only be affixed to a woolen tallit if they include techelet).
This learning should be in memory of Maran, HaRav HaGaon Ovadia Yosef, ztz'l.