I confess that I favor natural hackles, hairs, feathers, and dubbing. Naturally, I don't have more than a feeble justification for my preference in some areas; life would be boring if it was always bolstered by logic, rather than supported by pure, obdurate, obstreperous opinion.
Despite Veniard's longstanding offering of superb dyes, if we examine any dry fly plates from the 1920s-1950s we would be hard-pressed to find a color of hackle, feather, or dubbing that could not be found in the natural form. Natural hackles cover an amazing range of colors and tones, without need for dying. The same is true of feathers. Hairs were not often found in classic dries, but the various gray and brown furs used for dubbing in that period were available fresh from the pelt, undyed.
While at one time, a natural blue-dun cape was almost a thing of fiction, today they are readily available. There is really no reason to purchase a dyed material for classic dries.
OTOH, there are some reasons to keep all of our feathers in the natural state. Feathers, like the wings of butterflies, use "structural coloration"; that is, the tiny layers of chitin comprising the feathers are layered in such a way as to refract the light and produce the native color. Other materials, like furs, use pigmentation instead. To cover the structural color with a dye is to remove much of the innate translucence of the feather Dying may also radically alter the natural ultraviolet reflective characteristics.
OTHER, is my category for treated feathers and furs that do not appear treated, as well as honesty markings. Many white feathers and furs appear "whiter than white". This is because they have been treated with a brightener, typically TiO2 -- Titanium dioxide. TiO2 is used in most outdoor paints to absorb UV. It was even added to skim milk to impart a bluish color. TiO2 is fluorescent and the blue that it fluoresces makes it seen "whiter" to the human eye. However, the TiO2 absorbs UV, so the material would be seen as darker by the trout, which can see in the UV wavelengths. This was impressed upon me years ago when giving a lecture on UV vision at a FFing club. A member came forward with two white marabou streamers. He told me that his old marabou still worked but a recent supply was not nearly as effective with the trout. When photographed in UV, the difference in UV reflectance was obvious -- his new stock of marabou had been treated with TiO2 and reflected little UV, the untreated marabou was bright in the UV.
Do you like natural materials in preference to dyed?