This is copied off an ACNL Wiki site:
In September, trees will begin to lose their bright green color by turning a dull, musky green. In October, the trees will then turn a bright orange/brown color. Grass follows the same pattern as trees, by turning a dull green color in September, then a pink/orange in October, and even a shade of dull purple/gray as the months progress. Weeds also start to lose their color, and dandelions will cease to exist until next Summer and Spring. The trees will often gradually change color than rather it being an instant process.