Why does fall leaf color on trees vary from year to year?

FAQ
Question: 

Why does fall leaf color on trees vary from year to year?

Answer: 

The weather in early fall largely determines the intensity and duration of leaf color in those tree species capable of producing good fall color.  Best fall color typically develops when days in early fall are sunny and mild and nights are cool (but remain above freezing).  Long periods of cloudy, rainy weather or a hard freeze in early fall will mute fall colors.