1776 seems characterized by gobs of colorful, foppish finery, perched atop spindly, stockinged legs. But by the 1800s, we seem to be fully involved in more of a steampunk look, characterized by the color black, frock coats, and long pants. I have 2 questions: 1) Was the change really as fast and complete as it seems? 2) What brought such a fast, radical, change in such a short time?
First of all, in the 1700's only the aristocrats and the wealthy men could afford to wear flamboyant clothes, and by the second half of the 1700's even the aristocrats wore much more tuned down outfits. The middle class usually wore more subdued version like this or this while the working class wore even plainer clothes. The French Revolution killed the last colorful flamboyant clothes because people did not want to be associated with the decadence of the French aristocracy, the trend soon spread to other European nations and the US (you've probably heard of sans-culotte, they wore long pants instead of breeches). By the early part of 1800's long pants and some version of the three piece suit because the standard attire. Here's James Monroe with his cabinet illustrating this transition. Monroe wore something others might considered "outdated" while everyone else wore long pants and three piece suit, even though the colors of their suits were still brighter than later decades.
A very important figure in men's fashion is Beau Brummel, he established the standar look of tailored suit in understated colors.
you'll find lots more on the changes in men's fashion in this section of the FAQ*
*see the popular questions link on the sidebar or the wiki tab above
Thanks!
