Why Do The Leaves Of Trees Go Red And Gold In Autumn?

Leaves contain a variety of chemicals that only absorb some wavelengths of light within the spectrum of white light. Leaves that are green have that colour because of the chlorophyll inside them. This pigment absorbs red light and reflects yellow and blue light, so appearing green.

In autumn, deciduous trees, those that lose their leaves in winter, need to absorb some of the nutrients in their leaves before they drop off. It would be a waste of resources for the tree to discard leaves full of active chlorophyll, so a process called senescence is set in motion. As many nutrients as possible are reabsorbed and then pigments in the leaf are broken down. As the leaf dies, the chlorophyll is replaced by pigments such as anthocyanin, which is red and beta carotene, which is yellow.

This chemical cocktail of breakdown products in the leaves is responsible for the glorious autumn displays before the fall of the leaves for winter.

In short... They're dying.