There are two theories according to Jennifer Wells, Leaf Color Change:
Competition theory - states that nutrients are redistributed to important parts of the plant. Lack of nutrients in the leaves will decrease the chlorophyll synthesis thus the leaves change their color. The redistribution does not happen equally that's why some leaves are multicolored (see Ecology and the Environment at Cornell Blogs).
Mutual enhancement - states that specific hormones (senescence factors) work in conjunction and cause the color change.
All these happen under the influence of light - the absorbed dose of light is modulated by internal factors. The result is a reversed balance between carotenoids and chlorophyll (increase in carotenoids).
These carotenoids (fat-soluble) turn the leaf into orange and yellow depending on their concentration. There are also anthocyanins (water-soluble) which are responsible for red coloration.
Chlorophyll and its derivatives, if not carefully processed by organized biochemical pathways in green cells, can be toxic when illuminated.
According to: Howard Sidney Thomas, Leaf senescence and autumn leaf coloration. Check also Figure 3 from this link (on the last page).