"Color constancy" is the ability of our visual system to see an object as having the same color even when illuminated by different colors of light.  A human face, for example, looks roughly the same to our eyes whether bathed in the white light of midday, the greenish light of a typical fluorescent bulb or the reddish light of sunset.  Film, however, has no such capacity.  That's why most professional photographers shoot fall foliage only when it's bathed in direct sunlight (or direct sunlight diffused by clouds), which allows the film to render the foliage in saturated shades of yellow and gold.  Shooting fall foliage in the shade on a clear day usually doesn't work, because the leaves are essentially lit by the blue sky.  The color may look good to your eyes, but on film, yellow leaves lit by blue light turn a dull, yellowish-green.  The only exception occurs when the sky is full of brilliant white clouds that bounce the white light of the sun down into the shadows.  Even better is when those reflective clouds are bathed in the golden light of sunset, which bounces warm light into the shadows and gives the leaves a diffuse but vibrant glow.  A wonderful display of this atmospheric phenomenon is precisely what made this photograph of 14,150-foot Mt. Sneffels possible.  While it would certainly be trivial to mimic this look in Photoshop, my goal is always to make authentic records of rare and genuinely beautiful moments.