In general a salmon's flesh is dependany upon its diet, the higher amount of shrimp and similar crustaceans the redder the meat is going to be. Keoterne or however its spelled is present in crustaceans and is transfered to the flesh of fish when they are consumed.
There are exceptions for example the white kings of the Fraser that are unable to transfer this pigment to their flesh.
The reason why a salmon's flesh will be white as it matures is because the these pigments are transfered into the eggs and the skin.
Ever notice the redness of sockeyes and silvers and some runs of kings. The pigment that was present in their flesh has now been transfered to their skin for various reasons, particulary mate selection.