Where the Green Goes: Why Leaves Change Colour
The yellows and oranges were in the leaf the whole time. Autumn just takes away the thing that was covering them up.
A leaf is a factory. Inside it, a green chemical called chlorophyll catches sunlight and uses it to turn air and water into food for the tree. Chlorophyll is green, there is a great deal of it, and it hides everything else.
Two colours were always there
Alongside the chlorophyll sit other pigments — carotenoids, which are yellow and orange. They are in the leaf all summer. You simply cannot see them through all that green.
As days shorten and cool, many trees stop making new chlorophyll. The old chlorophyll breaks down and fades away. The yellows and oranges that were hiding underneath finally show.
And one colour that is new
The deep reds and purples in some trees are different: those pigments, called anthocyanins, are made fresh in autumn, often when days are bright and nights are cold.