There have been more than two strains introduced, some likely not well recorded. Then they have been mixed in hatcheries and spread all over. Then ... given popns of various mixtures have adapted to local conditions. Then ... there are iridocyte (the pigment cells in fish skin) adjustments to immediate lighting conditions, hormonal/behavioral circumstances, as well as health. Then ... there is individual variation, which can be considerable, as well as mutations. There are common traits shared by most fish of a species or strain within.
Talking about individual variation, here’s a neat one, an almost spotless brown, caught a couple years ago amongst other more “normally” colored/patterned browns:
WestCO, The purple marks distributed along the sides of all salmonids are called "parr marks". They are very prominent in fingerlings and beyond the “parr stage” (less than a year old and named for those markings) into adulthood. Large adults lack them, and likely do so after their first spawn (age 3 on average). Almost all but the very largest/oldest small stream fish tend to have parr marks, and I suspect this is, in part, bc the stress of spawning is too much on top of the normal stresses small waters dish out. Although I haven’t seen studies (except for some brook trout studies), I suspect repeat spawning on many small streams is rare. Productive small waters with good habitat during seasonal extremes likely produce more repeat spawners.
Here's a brown fingerling showing his prominent parr marks:
Interestingly, it also has the red in the tail and ventral fin margins already, but no spot behind the eye yet.
It seems hatchery fish lose parr marks early, as they lose color in general. Browns stocked as yearlings tend to be dull olive and have warped fins.
What the tomato juice-red fins are about I'm not sure but I've seen it a lot, esp back east, but some here too. I cannot associate it with anything but have assumed it’s related to immediate water conditions. I believe I see it most when water conditions are optimum and fish are in prime metabolic space. Possibly certain lighting (clear water but shadowed) plays a role as I seem to associate the two. But both ideas are guesses.
Most browns have that large spot behind the eye, but is most apt to be lost (or mixed in with other large spots) on very large fish.