Salmon change color to attract a spawning mate. Pacific salmon use all their energy for returning to their home stream, for making eggs, and digging the nest. Most of them stop eating when they return to freshwater and have no energy left for a return trip to the ocean after spawning. After they die, other animals eat them (but people don't) or they decompose, adding nutrients to the stream. Unlike Pacific salmon, Atlantic salmon do not die after spawning, so adults can repeat the spawning cycle for several years.
Learn more: Western Fisheries Research Center - Questions and Answers about Salmon