The colors we find in fly tying literature are baffling to us today. Where, for example, did they come up with the term "blue dun" or "golden dun"?
Examining the dictionary definition of "dun" gives us a starting point. "of a horse : having a grayish-yellow coat with black mane and tail" (Merriam-Webster Online)
The use of the term "blue dun" in fly tying predates the common use of the automobile. Needing a color description, it seems that the tiers turned to what would be most commonly understood and used horse colors as reference points. Thus, when documenting a fly pattern, "dun" would be broadly understood as being a "grayish-yellow." If in doubt, one only had to look out the window at horses going by in the street to understand the tier's intent.
Here then are some examples of duns:
Light Blue Dun
Medium Blue Dun
A herd of Blue Dun from Light to Dark
A Pale (Champagne) Blue Dun
It is only speculation on my part that our hackle colors are based the horse; what do you think?