Leaves serve as the solar panels for plants and part of what makes the process work is a chemical called chlorophyll. Chlorophyll is green and most leaves are loaded with it, and thus the majority of leaves are green, but what about the others? Another family of chemicals leaves carry are carotenoids, these chemicals are orange and are what give carrots their colour, hence the name similarity. For most of the year the carotenoids are hidden below the layer of chlorophyll, but when the chlorophyll breaks down over winter the carotenoids are exposed giving the leaves an orange colour.
Another common chemical family in trees are the anthocyanins; these chemicals have a red tint and are responsible among other things for the colour of a blueberry. These anthocyanins can be visible in bright and cold days in autumn and when they exist with chlorophyll the colour may be a warm bronze and when at a high enough concentration the anthocyanins will cause a purple colour as in Japanese maples.