From what I've read over the years, the reason salmonids have their orange colored flesh is because of the carotene found in their forage. If you consider that, after a trout gets over 12 inches they'll shift their emphasis to larger prey than the insects they grew up on, their flesh will tend to be whiter. It's the carotene in the shells of aquatic bugs that makes the smaller fish orange. At the sametime, in places where fish have an ample source of freshwater shrimp--aka "scuds"--their flesh will be more orange than white. If you notice, sockeye salmon have the deepest colored flesh of all the salmonids. This is because they graze on small things like krill--very small shrimp. Again, a good source of carotene. However, a salmon like chum is more strictly a predator, thus has a whiter, less tastey flesh. So, in the end, what makes a flamingo pink is the same thing that makes the flesh of a trout orange.