Carotenoids are produced by algae, plants and certain fungi.
Feeding fish higher-level animals in the food web (like crustaceans or fish that predate on crustaceans) concentrates the carotenoids and brings out more and more color in the fish, specifically red.
Whitish gray, like a lot of fish and bird meat. The pink comes from their natural diet, which they don't have in captivity, so farms dye the meat artificially so it looks like wild-caught salmon
If the color of the meat turns you off, wait until you think about the fact that it's a dead body
Just to add to this, the artificial colour is added to the fish's diet. They also adjust the colour depending on the market. For example, apparently Japan prefers a bright pink colour, while Italy and France prefer a less bright pink colour.