It's all due to the pigments. Most trees have green leaves because of chlorophyll, a photosynthetic molecule that contains pigment that reflects green light. During fall and winter, the cholorphyll is stored outside of the leaves so that it is not wasted when the leaves fall. Then more pigment containing molecules called flavonoids and anthocyanins are left behind in the leaves, creating an orange or red color respectively. So the concentration of certain pigments is going to determine the color of the leaves.