The orangey-brown you see on the leaves now is a puritan color. We call it russet. It was then called "Philly Mort," a corruption of the French feuille morte.* They preferred the restrained, subdued hues called sadd colors, which those who have read Albion's Seed may remember. Puritan hats were black. Black was otherwise considered a bit pretentious, or at least over-formal. Clerics adopted it as time went on, reflecting their increased self-regard. But for everyday, the colors which occurred in nature were considered acceptable, though even a few of those were suspect.
Consider, for example, the dull magenta which Harvard calls "crimson," and the dull blue and gray of Yale, or the dark Dartmouth green. And of course Brown has the color...brown. The colleges and universities in other parts of the country have more exciting colors. Here, it is rust, puce, tawny, forest green, and other somber shades.
Those are the old New England colors you could still find until after WWII. Immediately afterwards, all those gaudy golf/Bar Harbor/LL Bean colors suddenly became the mark of the moneyed, salt-water elite. I don't know why, but I suspect that the universality of the dull colors even among the poor here created a counter-reaction of adoption of shades that had heretofore been favored by the gaudy urban and ethnic poor. Just a guess on my part. But you will remember the preppy look of the 70s and 80s which tended toward pink and bright green. Or lemony yellows, Nantucket Red, and all the rest.
*There is a minority opinion that philly mort was an even duller, gray-brown color, but I am following the decisions of Plimoth Plantation on this.