Some, but not all shrimp, are grayish when they are raw, but turn pink once they’re cooked. It makes sense to guess that this is because some chemical compound with a red color is being produced once you add heat. What’s actually happening, though, is that the more intense pigments in the shrimp’s shell are decomposing with heat, while the compound responsible for the red color is more stable. That red molecule is called astaxanthin, and it’s not only found in shrimp shells but is also the reason that salmon meat is red.