Visitors from east of the hundredth meridian viewing the fall colors in Rocky Mountain National Park often remark on an interesting difference between the regions: the colors of the Rockies are truly singular, that is, they are all yellow. Gorgeous expanses of yellow aspen (Populus tremuloides) color the mountain sides, contrasting firmly with the dark green spruces and firs. The orange, red, and purples of the east seem absent.
The mountains are especially good places to view fall color, because the steep slopes display the spectacle as though it were a painting hanging on a wall. From campgrounds and picnic areas, trails and roads, the masses of aspen lay bare all the golden splendor of fall's foliage.
But if we look more closely, other fall colors reveal themselves, although more subtly than the grand expanses of mountainside aspen groves. Cottonwoods - which are uncommon in the park, but frequently seen along roadsides in adjacent areas - often turn a beautiful yellow-orange.
But for most of the other colors, one must look more closely, maybe down rather than up, perhaps on hands and knees. For most of the plants of the high country serve up their hues in small portions.
The autumn colors in leaves are produced by an interestingly subtractive process. In summer, green chlorophyll masks the colors of several other pigments that exist in leaves, pigments that, like chlorophyll, assist with photosynthesis. These yellow, red, and purple pigments - carotene, xanthophyll, and anthocyanins - produce the bright fall colors, but only after the chlorophyll wanes as temperatures cool and days shorten.
As autumn proceeds, even these hardier pigments ebb, and leaves become brown, gray, or black.