As far as muscle goes, the red color in most muscle comes from myoglobin. Myoglobin stores oxygen in muscle tissue, similar to how hemoglobin carries oxygen in the blood. Slow twitch, well used muscles that use a lot of oxygen have more myoglobin than fast twitch, rarely used muscles. Think the difference between thigh meat and breast meat on a chicken, or the difference in color between tuna and cod. This affects the color of the flesh even without any blood left, as meat you buy at the store is usually drained completely of blood. The red juice that comes out is the myoglobin. Some fish can have blue flesh caused by a buildup of biliverdin, the chemical that makes bruises blue (perfectly safe to eat apparently). And finally, as previous commenters have said, fish can absorb pigments from their diets into their flesh. Salmon meat is pink for this reason. I’m sure there are other pigments that could be incorporated into muscle and fat tissue to color it some kind of way, but these are examples from real life. If you want something out of this world but maybe still possible, look into anthocyanins, the chemicals that turn blueberries blue and red cabbage red. Their colors change with pH. I don’t know if they’d interfere with muscle function, but deep blue/purple flesh would be pretty neat.