It’s all about Chemistry:

Not all leaves turn vivid colors.  Only a few species of deciduous trees produce the beautiful colors this time of year.  In Minnesota, most notably, we have aspen, maple and oak.  Soil moisture, precipitation, temperature and light all contribute to fall color.  Light, the lack of it, is the main agent.

As autumn days grow shorter, thus less light, chemical changes in deciduous plants cause a “corky” wall to form in between the twig and leaf stalk.  This “corky” wall or abscission layer eventually causes the leaf to drop.

The chemical change seals off the vessels that supply a leaf with nutrients and water.  It also blocks exit vessels, thus trapping simple sugars in the leaves.   Reduced light, lack of nutrients and not water add up to the chlorophyll (which makes the green color) to die.

Once the green is gone, other pigments take over.  Carotene (yellow) and anthocyanin (red) exist in the leaf all summer but are overpowered by chlorophyll.  The brown in autumn leaves is a result of tannin.

Sugar trapped in the abscission layer is largely responsible for the vivid colors.  Sunlight acting on the trapped sugar also helps to manufacture anthocyanin (red).  This is why colors on bright fall days are crisper and duller or more pastel during times of rain.

A wet growing season and a dry autumn filled with sunny days combined with cold frost free nights helps produce the most vibrant colors of fall.

This is a helpful graphic to sort out all the information above: