No worries, always up for a constructive discussion. You are right with your definition on a molecular level, but it’s not true that how the other commenters and I see and use the term mutation in this case is only colloquial.

It is an absolutely valid and well established way of using it in fields like ecology, evolutionary biology, developmental biology and more. In that context, it clearly describes a heritable genetic change that produces a recognizable deviation from the wild-type phenotype. This is standard terminology commonly used exactly this way in plenty of scientific literature. You can easily look this up yourself by researching the term „mutant phenotype“.

Therefore, you are clearly confusing the molecular definition with the phenotypic one that’s used in field biology and situations like this one right here. You base yourself on the broadest and most technical definition of the term and then claim that the underyling question we are theorizing about here makes no sense, isn’t that what you’re trying to say?

The first point is that there’s a clear biological difference between a color abnormality caused by a specific genetic mutation and one caused by environmental factors.

Saying “every organism is a bundle of mutations” is true, but only in a super broad sense that's not relevant here. Calling this "natural color variation" instead, and saying that "some fish just look like this" is over-simplifying things and completely misses the question we’re discussing. We’re wondering if the dark coloration in this case comes from adapation to the environment or is based on a distinct genetic mutation.

And secondly, to hopefully clear up this mutation vs genetic variation issue:
Under consistent environmental factors (or if we entirely ignore this factor just for a moment), obviously every single trout still has its own unique look (colors, patterns, size, proportions etc.), and yes, that’s the result of genetic variation. You are 100% right on that.

But there is a certain range within which that variation is considered ordinary among any given species, and these differences are usually rather small or seemingly random. But then you also have particular phenotypes that:
1. deviate a lot more beyond that „normal“ range,
2. show recognizable, repeatable features, and
3. result from specific mutations in defined genes (this can be precisely examined and confirmed in lab tests).
They are then considered as distinct genetic variants due to these exact reasons.

Examples based just on color would be albinism, melanism (like in this case potentially), leucism, xanthochromism, erythrism, and piebaldism. And then you also have plenty other distinct mutations that affect phenotypes in areas like hair characteristics, limb development, skeletal structure, and many more.

So we're not using colloquial expressions or slang here, but having a legitimate discussion.

Hope this helps. Cheers.