"The two main groups of pigments that give autumn leaves their colors are: Carotenoids, which produce yellow, orange, and brown colors; and Anthocyanins, water-soluble substances which are produced when there are high levels of sugars in leaves or fruits under bright light conditions. Bright days and longer cooler nights, with adequate, but not excessive rainfall, are generally considered to be the catalysts for great fall leaf color.
The U.S. Department of Agriculture has a good, readable explanation of “Why Leaves Change Color” online, for all of us who have never tired of asking “why?” Whatever the color, do heed the good advice on this site and others to rake and compost fallen leaves. They also make an excellent mulch when left in place where they fall. Tiny micro-organisms will put them to good use replenishing the all-important organic content of the soil."