Why is Colatura di Alici treated as an alternative name for anchovy essence? From wikipedia:
Anchovy essence – used as a flavoring for soups, sauces, and other dishes, anchovy essence is a pink-colored, thick, oily sauce, consisting of pounded anchovies, spices and other ingredients.
Whereas Colatura, again from wikipedia:
The sauce is a transparent, amber-colored liquid, produced by fermenting anchovies in brine.
It seems as if colatura is closer to garum than to anchovy essence - something I can vouch for having made spaghetti with colatura many times.