Then, about a year ago, I stumbled across a card of original Chadwick's 477 and bought it for the princely sum of $5. Sometimes an angler just gets lucky! A lengthier account of this windfall is described at: http://flymphforum.com/viewtopic.php?f=5&t=6123.
With a card of the real deal in hand, I've patiently visited yarn shops and online merchants during the past year searching for a good color match. As described below, I recently found a very good one. While not identical in color and texture, it is closer to Chadwick's 477 than anything I've seen so far.
But first, a comment about color and the Killer Bug. I am an amateur student of fly tying history, and my search for a Chadwick's substitute is about recreating history, not about crafting a more effective Killer Bug. Of all the features that contribute to a fly's success, color is well down on my personal list of things that are important. I suspect that most, if not all, of the previously discussed Chadwick's substitute yarns work well. As with most flies, I believe a Killer Bug's success is mostly determined by how it is presented, not by how closely it matches Frank Sawyer's canonical color. The search for a Chadwick's substitute was in the interest of history and in providing other anglers with a yarn whose color replicates as closely as possible Frank Sawyer's original. Then, when we fail to replicate Sawyer's nymphing success, at least we can't blame the color of the fly.
In photos below, I've worked hard to make the color balance in my photography and on my computer monitor as close as possible to the real item. Your monitor may vary. The most useful photos are those in which both the Chadwick's and substitute yarns are in the same photo. That eliminates potential differences of camera settings, lighting, exposure, etc.
Perhaps the most convincing demonstration of the substitute yarn's color match to Chadwick's 477 is this photo:

Five of these Killer Bugs are tied with original Chadwick's 477 and five with the substitute yarn ("Berroco UAF1214", see below for details). Kind of difficult to tell them apart, isn't it? The five flies on the left (marked with a bit of red thread at the eye) are tied with Chadwick's 477, and the five on the right are tied with Berroco UAF1214.
I learned the hard way to mark Killer Bugs tied with each yarn. While investigating various yarns as possible 477 substitutes, I laid on my tying desk several Killer Bugs tied with Chadwick's and several tied with Berroco UAF1214. I inadvertently brushed them together, causing the flies to be mingled. I could no longer tell them apart! They all looked more-or-less the same. Those precious Killer Bugs tied with Chadwick's 477 were instantly lost to ambiguity!
Realistically, no two Chadwick's-tied (or UAF1214-tied) Killer Bugs are identical, because the exact blend of colored fibers along a several-inch piece of yarn varies from piece to piece. Individual fly-to-fly variation among Chadwick's-tied flies and among UAF1214-tied flies is as great as any differences between the two groups. Upon realizing that, I knew the search for a substitute yarn was over.
Here is another example of the color match. Three Killer Bugs tied with Chadwick's on the left and three with UAF1214 on the right:

And another example. Three of each, but I've lost track of which are Chadwick's and which are UAF1214. (These are the flies that were inadvertently mingled.):

Close-up views of Killer Bugs, both wet and dry:




A comparison of the Chadwick's yarn (bottom) and UAF1214 (top) as each appears on a card:

When viewed in daylight (important for seeing the true color), Chadwick's 477 is a slightly grayish, slightly pinkish medium tan, which Sawyer described as "a fawn background that has a definite pink tinge". Berroco UAF1214 is similar, but the tan is somewhat darker, and its reddish color is more of a brick red than a pinkish red. Both yarns are finely textured, with UAF1214 being slightly larger in diameter and more tightly twisted. Chadwick's is 85% wool / 15% nylon, while UAF1214 is a 70% wool / 30% nylon. Both are twisted 3-ply (Chadwick's) or 4-ply (UAF1214) yarns. I use a single ply of UAF1214 to tie size #14 Killer Bugs. Chadwick's yarn has a softer fuzzier surface, and I therefore pull UAF1214 lengthwise a few times between my thumbnail and index finger in order to pull some of the fiber ends away the yarn's surface. I also unwind the twists of a single ply of UAF1214 before wrapping it as Sawyer described over an underbody of red wire.
The yarn is "Berroco Ultra Alpaca Fine", color #1214 ("Steel Cut Oats"), dye lot #2J9711. [Note: this is not the same as Berroco "Ultra Alpaca", "Ultra Alpaca Chunky" or "Ultra Alpaca Light" yarns.] I have no idea how many different dye lots of Berroco UAF1214 are kicking about, but I've handled three. They are similar, but not identical. Dye lot #2J9711 is slightly more reddish than the other two (dye lots #001 and #1101). The yarn is made in Peru and available from quite a few online yarn merchants. One 400m skein will tie an infinite supply of Killer Bugs for you and all your friends.
The Berroco Ultra Alpaca Fine #1214 yarn label:


A faux Killer Bug in action about 10 days ago.


The fly worked well in the spring creeks of Wisconsin's Driftless area, where scuds and cranefly larvae are quite abundant.
For those of you who might be Chadwick's chasers, the photos below compare Chadwick's 477 to several of the more often discussed substitute yarns. In each photo, Chadwick's is on the bottom.
Chadwick's and Jamieson's Shetland Spindrift #290 (Oyster), which is the yarn of a "Utah Killer Bug". Oyster is very pink!

Chadwick's and Lion Brand Wool-Ease #403 (Mushroom). Wool-Ease is 86% acrylic, 4% rayon, and only 10% wool. Its base color is a pretty good imitation of Chadwick's 477, but only after the large, dark, coarse fibers are removed one-by-one with forceps. Being mostly synthetic, it doesn't seem to change color when wet quite like yarns of high wool content.

Chadwick's and Paton's Classic Wool - Worsted #229 (Natural Mix). Paton's is quite tan and grayish tan in color, with patches of darker gray. It contains little if any of the pinkish tinge.

Chadwick's and two substitute yarns dyed by Tom Waits, whom I get to see occasionally (but not nearly often enough) in Wisconsin.

My search for a Chadwick's 477 replica is mercifully over. Instead, I'm now on a jag to find authentic ingredients of Tup's Indispensable dubbing. Most of the materials are pretty straight forward, but not so with the creamy yellow wool harvested from the testicles of a white ram. That's a tough one! As with Chadwick's 477, however, the challenge appeals to my historical interests. I almost scored some of the wool recently, but the sheep farmer (father of a friend) reported, "The ram didn’t cooperate". I doubt that I would either. Anybody got some?
Cheers,
Phil Anderson
Email: Third-nospace-Meadow aat outlook dawt com (replace or delete as needed)