It is common for Oscars' color to fade and brighten back up. Mine always seemed to fade at night when the lights were off. After the lights were back on for a while, they would brighten up. Beyond that, stress and excitement would certainly cause changes. My Oscars were always in a tank of their own and I always had two at a time. Perhaps they didn't have enough going on around them to cause frequent change. Your tank has a lot going on and, more than likely, it's just a trait for that particular fish. You should continue to be observant, but I wouldn't be too concerned at this point. If he was hiding and not eating, it would be a different story.