They adopt the coloration and the behavior that goes with it to defend something that is defensible, like predictable patches of food or cover. So colored fish show they are ramped up and ready to fight. The "brats" attack other fish entering their areas either driving them off or distract the others enough so they can not eat as effectively. In my larger tanks, a brat can almost exclude other sunfish from an area that can approach a square yard in size. My largest Hand-paint Bluegill (a female) outright attacks the other Bluegill that approach area under light where she gets first dibs on flying insects that either fall into water or come down close enough her to jump up for it. She only backs off, and just a little, when she is satiated. When we run feeding trials, each tank will have a couple brats that attempt to drive tank-mates off as fish are hand-fed. Our protocol with hand-feeding insures that all get most of what they want. What can be a problem is when we under feed or use an automatic feeder that meters out feed too slow. The brat can exclude other fish from eating setting up for size differential pretty quick.

You can see all this in ponds as well. Green Sunfish juveniles will defend areas near a nesting colony where they sneak in to eat nest bound broods. Bluegill will defend patches of bottom where they can pick of items like blood worms an tubificid worms they treat almost like gardens. I have seen Bluegill defend cover patches like a clump of plants where fish is apparently able to watch more diligently for zooplankton floating by. Sneaker males sometimes will also adopt the brat approach coloration.

Being a brat may have cost going with it. That would be why not all do it. Other sunfish species do it although much of color pattern change differs. Juvenile Smallmouth Bass and Spotted Bass do it to an extreme and the tri-colored tail is key to the display.

Aquaculture
Cooperative Research / Extension
Lincoln University of Missouri