I've been thinking much on this idea this week. I was forced to medicate some tetras for an illness. While being treated their colors changed to be lighter and more even. I believe stress from illness had caused a more contrasting pattern.
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Nice picture. It reminds me of a similar experience. An African cichlid became a terrible bully so I moved him into the pike cichlid tank. All went well until the pikes grew big. Similar in size difference to your picture. Then his bullying tendencies returned and the pikes were forced to return his aggression. One on one the Cynotilapia punished the pikes. But the group worked cooperatively and soon returned a just reward. They moved him around the aquarium like a decoration. Pikes do sometimes play with things. Even still, he would have survived but he just wouldn't stop.
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I'm thinking you're right. The color change was a response to a change in the environment. The new fish made him uncertain for a time.