Hi Rick,
Thought provoking article. I have different colors of thread, and usually tie the fly using the color of thread the pattern calls for. I have never really questioned this, just did it, although when I first started out I had only a few colors, and tried to pick a color as close to what the pattern called for as I could.
However, I know that some tiers tie most or all of their dry flys with black, or black on dark flies, and one light color on all of their light color flies. They use only two colors of thread. This may be just fine, I don't know, and have never tried it. If just two colors of thread, or maybe just a small set of 3 or 4, are all you really need, it does simplify things.
One reason for using different colors, at least for me, is my own notion that a particular fly should look a certain way, and that way is normally to match the picture in the fly tying pattern book.
What ought to count for more is how well the fly works.
At any rate Rick, very good article.
Regards,
Gandolf