Aspen leaves turn shades of yellow and sometimes red in late September and early October, depending on elevation and latitude. At 9,000 to 10,000" elevation, in most years, the leaves turn in late September and are gone by the first week in October. In the Southern part of Colorado, such as the San Juan Mountains, the leaves at those elevations turn in the first week of October. The type of color, brilliant to dull - is affected by wet or dry seasonal conditions. A very wet Summer with heavy rainfall throughout usually results in a leaf rot that forms on the leaves and dulls the yellow colors in the Fall. A dry Summer, or just a dry September, stresses the trees and often results in brilliant yellow, red, and orange colors. Backlit, the brilliant yellow leaves have a gold appearance, called "Aspen Gold". My favorite areas to photograph the changing Aspen in Colorado are: Boreas Pass Road above Breckenridge, Maroon Bells above Aspen, San Juan Mountains near Dallas Divide, and Crystal River above Marble. Every Autumn in Colorado is different - in the hues of yellow and the clear or cloudy weather. Autumn is the season for scrambling from one place to another - I have just a two week window to capture the color before the leaves fall, unlike the Spring, Summer, and Winter when I have several months to work with. After the leaves fall, dropping from the branches or blown by the wind, I am filled with anticipation of the Winter, the opening of ski season and whipped cream snow on the Pine trees to photograph.