It is the green pigment in the leaves. During the spring and summer when trees are growing, nutrients continually add chlorophyll to the leaves. Because the dominant pigment is green, so are the leaves. Leaves begin to change color in fall when trees become dormant and shut off the supply of chlorophyll. No chlorophyll, no green .
Carotenoids are the chemical pigments that produce yellow and orange.
Carotenoids have always been in the tree leaves. The yellow and orange pigments are simply overwhelmed by the green in spring and summer. Take away the green and you reveal the yellow and orange. Anthocyanins are the red pigments that are responsible for red apples, strawberries and cranberries. Where carotenoids are always present in leaves, anthocyanins develop late in the season.