Whilst gnu is correct in linking daylight to leaf-fall, according to species there is considerable environmental input when it comes to colour. Water availability, or lack of it, in summer and low autumn temperatures can have an impact on colour. A tree sheds leaves in autumn because they would be a liability in winter - too little light to help them to do their job, too much exposure to damage caused by severe weather, worsened physiological drought caused by water loss from leaves in hard frosts, fully-laden trees more susceptible to being blown down, etc. Trees that keep their leaves in winter have strategies such as waxy leaf cuticles, thick epidermises, sunken stomata and reduced individual leaf area (though more actual leaves, as in conifers). Their branches may be so disposed as to shed snow more easily. Obviously, not having to replace all your leaves at once in spring saves energy. Evergreen trees do lose leaves but in a constant trickle. Leaf-fall is also the way trees get rid of some waste products such as tannins and calcium oxalate. Leaf-fall is a tree's way of going to the toilet. They get away with doing it in public in a way that neither you nor I ever could.