The scientific explanation for color changes is supposed to be the individual pigmented cells (melanocytes, I believe they are called). The pigment in these cells can expand or contract very rapidly, apparently in groups, making that area of the fish darker or lighter. Smallies, when you see them unstressed in clear water, are usually almost unmarked, a uniform color with no bars or spots. The dark bars on them that we often think make them look so pretty are often signs of stress, and they can show up in the short time between first hooking one and landing it. Spawning fish, especially the males, will also be heavily barred, so the barring is also a sign of arousal.
The fish can also get uniformly darker or lighter in response to water clarity and light levels. The odd thing to me is that bass in muddy water are usually rather washed out in appearance, with no markings and light color. Another interesting thing is that wintertime fish are almost always much lighter in color than summertime fish. I believe the reason for this is that in the winter, the bottoms of most streams are much lighter in color because the algae that covers the bottom in the summer is gone and the gravel is cleaner. The lighter wintertime fish, and smallies in very clear water over a clean bottom even in the summer, will almost always have some bright red in their eyes. The reason for this is that those pigment cells are also found in the eyes, and if the fish is dark, the eyes will be dark as well because the dark pigment is overpowering the underlying red, while if the fish is light, the underlying red in the eyes shows.
What all this does not really explain, though, is the difference in color, not shade (darkness or lightness) in the fish in the original post. Smallies will almost always, as a “default” color, match the color of the water or the bottom of their native streams or lakes. In tannic stained waters they will usually have those rich brown colors, in clear, deep water they will often be that more greenish color. In Big River, the stream I grew up on, in the stretches that were choked with lead mine waste, the fish matched the color of the mine waste. Unless covered with a lot of algae (and since the waste is pretty sterile and unstable, it is seldom covered by much algae) the mine waste is a grayish color. Smallies in those sections were usually a light gray green in color, matching even that “unnatural” bottom very well.
And those really, really black ones? I just don’t have a clue why they are that color. I’ve tried for many years to get a good photo of one that’s so black, but they are almost always small, under eight inches, and when you DO catch one, in the very short time it takes to get it in and snap a picture, it has already lightened up considerably.
