Trees and plants use pigments in their leaves to trap light energy, which they then use to produce sugars. The main pigment used by most plants to capture light energy is known as chlorophyll, and gives leaves their green colour. When deciduous trees are preparing for winter, they allow the chlorophyll in their leaves to break down into amino acids which the tree recycles and stores, ready to use in the growth of new leaves in the spring. As the chlorophyll breaks down, it allows other pigments present in the leaves to become visible.
The yellow and orange colours are the result of pigments known as carotenoids. Carotenoids are present in leaves all through the growing season, but their colour is usually masked by the green chlorophyll. 
The reds and purple colours are the result of pigments known as anthocyanins. These are not present in the leaves all year around, but are produced in the autumn as the plant is recycling its chlorophyll and allow the plant to capture light energy for a little bit longer.
Combinations of these pigments add further to the range of colours to be found in autumn leaves and produce autumn's palette.
In addition to vast array of colours (the only colours that are missing are the blues, but on a nice clear autumn day, the sky makes up for that), I love the array of patterns the different colours make on individual leaves - each one is uniquely different and rarely a single shade. Here are a few for you to enjoy :)
