The beautiful reds, yellows and oranges that light up both our natural landscape and our gardens are brought on by several changes. The days are shorter and the nights cooler, even before the first frosts. There also tends to be additional ground moisture at this time of year. As the tree begins to stop food production, the amount of green pigment in the leaf, which we know as chlorophyll reduces. The carotenoids (which produce the orange-yellow colors) and anthocyanins (which produce the reds and purples) become the stronger pigments, overwintering the green chlorophyll. 

This is what gives us the gorgeous, varied tints that we know as autumn color. However, the autumn color is variable and relies heavily on some late summer rain. It is probably the most difficult season to predict, as it is often spread over a period of at least three months and it's never the same from year to year.