Why do leaves change colors in the fall?

The shortening daylength causes the trees to change their logistics of sugar
usage and start moving sugar to the roots for winter storage. Some of the extra
sugar is converted to anthocyanin and other pigment and remains in the leaves.
Some of the yellow color is probably caused by chlorophyll becoming paler. Also
keep in mind that in some trees there was some pigment present all summer, but
it was masked by the green chlorophyll.
Another factor is that anthocyanin production is enhanced by sunlight during
the day and cool temperatures at night. You will notice that houseplants with
red flowers or red leaves color up more if they have bright light and cool
temperatures.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)