Why do raw prawns turn red after sitting in vinegar?
The prawn has proteins (meat, muscles).

Beef is red because the muscle proteins in it have a molecule that binds oxygen called myoglobin (whereas our red blood cells have hemoglobin that allows them to carry oxygen better).

In fish you know the meat is white because there is no myoglobin present (in the muscle proteins), but another molecule that still binds oxygen but doesn't look red. 

When you add heat or acids (vinegar) to the meat, the proteins get denatured, change their shape and their colour too. You get red shrimps by heating the proteins in them.

Hope this helps