I agree that the various strains of browns have different coloring and density and sizes of spots. The "Marble trout" of the Balkans (which I've only seen in pictures) have a vastly different spotting pattern and density than browns from elsewhere. But I also believe that the conditions in which the individual fish live have a lot to do with their coloring.

Eastern (US) browns seem to be darker overall than those from western rivers. A lot of the good to better brown trout water in the east has a substantial amount of shade, while (my experience with) western waters shows the western to have fewer and less dense overhead vegetation. You'll get darker fish in the east b/c they live in dimmer surroundings.

I am of the belief that the amount of light the fish get - taken in through their eyes - governs how their body colors itself. I saw a very clear example of this some years ago. There was a large-ish brown who lived in the tangle of stuff that had built up around a gabion on the bank of the Little Lehigh. At some point, this fish had sustained an injury to one eye which apparently left it blind on one side (the right). That side of its body was almost black while the other (left) side had more or less normal coloration. It habitually rose and took flies on its left but not its right side.

I don't know whether there's any "hard" science to back it up, but I saw it and believe it.

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