So we all know the magical green chlorophyll, but leaves also contain carotenoids. As chlorophyll fades the golden oranges of carotenoids start to shine through. You may recognise one carotenoid, beta-carotene, from carrots. Another carotenoid, xanthophyll, is responsible for the beautiful bright yellows. The leaves of some trees also produce anthocyanin as Autumn begins that gives beautiful deep reds.
What a lovely time of year.
Also see: what causes seasons.