Green leaves show that the tree is still making food using photosynthesis.
Orange and yellow leaves show that the chlorophyll process is breaking down and orange and yellow pigments, called carotene and xanthophyll cause the leaf to turn orange and yellow.
Red leaves are caused by anthocyanins, another type of pigment in the leaf. As the chlorophyll process is breaking down the anthocyanins pigment starts dominating and turns the leaf red. Anthocyanins also protect the leaf from sun damage.
A brown leaf is caused by waste products trapped in the leaf.