Found this interesting,
In the wild or on the farm, the color of a trout's flesh depends on its food supply. A naturally occurring pigment called astaxanthin, found in many crustaceans, accumulates in the flesh of salmon and trout that eat them, and this pigment is the source of the orange-red color typical of salmon. Wild rainbow trout in fresh water eat a mixture of insects and small crustaceans, which gives the meat a light pink color. Their seagoing cousins, salmon and steelhead (the latter a rainbow trout that has migrated to the ocean), eat a higher proportion of crustaceans, mainly small shrimp and their smaller relatives called krill, and have resultingly darker orange meat.
The vast majority of farmed rainbow trout get a diet based on grain and fish meal, and they have pale-colored meat that cooks up to an ivory color.
