Deer, Elk and antelope can be different colored as they occur in different locations. So goes it with fish. California halibut take on the appearance of the bottom that they lay on. In mud they look like their habitat of dark mud and at Catalina they are lightly speckled and resemble the gravel bottom that prevails. Most all fish control their colors.
Trout really change their appearance during their spawning period as with the salmon.
But with the browns I have noticed subtle differences other than spawning changes. Once my dad caught a 35 inch brown trout out of Lone Pine Creek that the local papers named as a loch leven trout. This was popular name for a brown trout during those days, a name that has since been virtually forgotten. As a matter of fact I had all but forgotten about it until Jed told me this story. In Scotland there was series of lakes that the Scots simply named lake one, lake two, lake three, etc.