Sea-going sockeye salmon (on top) have iridescent silver flanks, a white belly, and a metallic green-blue top, giving them their "blueback" name.
As sockeye salmon return (below) to their freshwater spawning grounds, their heads turn green and their bodies turn bright red, hence their other common name in Alaska is “red” salmon. Source: NOAA
Source: FIS by SeaoodMediaGroup | Click to enlarge it