sam2- why is it shver?
patur aval assur-
The same way the yerushalmi is mashma it is green (yam domeh lasavim) the bavli is mashma it is blue(yam domeh larakia). So either you say it is a stira and the bavli is more authoritative (especially since we have additional rayas from kla ilan and historians that it is blue) or you are not gores the yerushalmi like the gra. Al kol panim the majority of the rishonim pasken it is blue.
also none of this is soiser the murex as the chilazon, since it is technically possible to dye green from it.