Hypermelanistic (black) Eastern garters populate some areas of Ohio, and I caught a red-phase Plains garter in my back yard here in Omaha.
Whether a color mutation is detrimental depends on what the color mutation IS. Albinism is going to make it harder for animals to thrive, because they stand out against the background. Likewise for hypomelanism, but only up to a point. Mutations that interfere with pattern may also disrupt an animal's camouflage, but not always. Hypermelanism is probably the most likely mutation to find thriving in the wild, and you see hypermelanistic animals in a wide variety of species doing exactly that.
Some of the mutations we love in ball pythons probably cause no problems in the wild. Yellowbelly (so long as it's the single-gene form), cinnamon, etc.
All of our beloved mutations were found in animals caught in the wild, not one has spontaneously appeared in captivity, to the best of our knowledge.
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