I always love the autumnal colours with the leaves changing from green to red, yellow and gold to shades of brown. This is due to the pigment chlorophyll decreasing when the daylight hours shorten and temperatures cool.
Carotenoids are present in leaves the whole year round, but their orange-yellow colors are usually masked by chlorophyll. The reds and purples come from another group of pigments called anthocyanins. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer. Check out the palette at colourlovers