Most fish have the ability to alter their color based on surroundings/mood/whatever. No where near the ability of say squid or an octopus, but they can still change colors, usually lighter or darker based on stress or environment. My bet is that there was some nerve damage done (either before the death or related to cause of death - head bashed in?) which led to the partial color change.
I've seen this in freshwater aquarium fish (when still alive) that have nerve damage resulting in half of the fish being black with a very distinct line where it starts and stops as they lose control of their color altering mechanism.