Fish flesh can run the gamut from ruby red (yellowfin tuna) to electric blue (lingcod), and NPR's The Salt blog has a handy explainer on the reasons behind those vibrant colors.
Yellowfin tuna, for example, is the "Michael Phelps of the fish world," and needs a lot of oxygen to go to its muscles. A protein called myoglobin stores that vital oxygen and also serves as a pigment, making the flesh pinkish red. "It's true of land animals, too: If they're walking around a lot, they'll have more myoglobin and their meat will be darker," says zoologist Bruce Collette.
Chinook salmon eat krill, which are filled with pigments called carotenoids; those carotenoids then turn the meat orange. Lingcod are bottom-dwelling fish whose blood serum turns blue due to a bile pigment called biliverdin. Then there's boring old halibut, which is off-white because it is a rather slow fish (it's no tuna) and often just chills on the sea floor. Read more about what makes fish flesh so colorful (or not, sorry halibut) at NPR.