Carotenoid is an organic pigment that is found in leaves. Depending on the concentration of this pigment, leaves can be red, orange or yellow.
Leaves are green in the summer months because there is an abundance of chlorophyll within the leaves. In autumn, the level of chlorophyll decreases because of the decreasing light levels. Leaves with a high concentration of anthocyanins will appear red. Leaves with a mixture of anthocyanins and carotenoids will appear orange. Leaves with a high concentration of carotenoids and a low concentration of anthocyanins will appear yellow.