Color of things important at many levels. When I try to catch larger groups of fish in spot here nets are not ideal in terms of shape or size, then I use two nets of differing color. One net is neutral / tone similar to back ground. The other is loud as in black or white. The loud bet is moved about and the fish retreat to the other net using it as cover.
A relatively new color change that is mood related for me pertains to how Bluegill color up to defend territories that have nothing to do with nesting.
A few years back I had four crystal clear ponds each stocked with 600 Bluegill representing Northern Bluegill, multiple stocks of Coppernose Bluegill, Hand-paint Bluegill, what I think were Southwest Bluegill, and all possibly crosses. It was like a trip to the candy store being able to see the color differences that could be discerned from 20 feet away with ease.
Mood is probably the wrong word as it implies a human type emotion.
But some types of fish really put on a show. When approaching other fish of their species, when they are in the mating mood. When they are approaching divers or divers are approaching them. When danger is nearby.
Filefish come to mind. Scrawled filefish have irredecent blue stripes they can turn on and off, go from light almost washed out to dark. White spotted filefish are blocks of orange, white and brown but can instantly go to a white spotted background and often flash their spots at divers. A very curious fish that if feeding can be aporoached closely while they change colors and keep a wary eye on you. Both of these species in the two or three pound range. Then the tiny slender filefish that can get up to 4" but more commin size is 1 to 3". Masters of camouflage, most divers never see them unless pointed out. They are solid color and only go from light tan to dark brown, but as they float in amongst a grogonian they blend in so well you can be looking directly at them one moment and loose them the next. If they drift from one color (what looks like a plant but is actually an animal) to the next they can change shades like flicking on a light through a dimmer switch. The transition blends in perfectly from one background to another.
The one really cool thing about staying in the same place for an extended time (vs diving a reef once and leaving) is you really start getting into the fish behavior. The reef in front of our room is as familiar to me as my own ponds. I dive it at least once nearly every day. So I see many of the resident fish over and over. Some species, like some of the puffer family, will get almost sociable with divers. I played hide and seek with one one time while other individuals are standoffish. One resident turtle is so used to divers my wife was able to take macro pictures of its head while feeding with the camera 6" away. It would look up at us once in a while then go back to ripping at something under a coral head with its beak it was intent on eating. I see that turtle probably once a week as it passes through the general area. The reef out front is like my own back yard with the pond.
Treated to a pod of dolphins passing through at breakfast this morning.
Humans are not special when it comes to mood, except with possible exception to think they are the only ones to exhibit it.
I watch fish in culture setups and streams pretty close. In culture setting, the sunfishes have really tight home ranges and often easy to read pecking orders. In much more complex stream settings their home ranges are much larger and they switch behaviors as they progress through different microhabitats and encounter eats, predators or competitors.
Warmouth are capable of exceptionally fast color changes as they move through tunnel system looking for prey and when encountering other Warmouth. The cool social behaviors do not show up in a tank setting unless tank is really big with lots of structure.
Aquaculture Cooperative Research / Extension Lincoln University of Missouri
Mood may indeed be the right word. Don't want to get into semantics "what does is mean" but if interested look at this wiki link to emotions which are a basis for mood. Mood often relates to emotions some of which are physical responses to stimulus. We have so much to learn about animals. The problem is that we tend to attribute human concepts to actions of animals when in truth there may well be a scientific basis.
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