Green leaves like grass clippings retain more Nitrogen than dry leaves. Dry leaves do contain a small amount of Nitrogen and will decompose naturally themselves but it would take a much longer time without human intervention by adding additional Nitrogen sources. "Greens" are not necessarily green in color, they can be other things naturally high in Nitrogen like coffee grounds, tea or manure. Bacteria do most of the work in decomposing organic matter and they consume carbon compounds and Nitrogen compounds for food and reproduction. Adding "greens" is just giving them a greater supply of Nitrogen to help them grow and reproduce faster than they would in nature.
Elizabeth is right about shredding the fall leaves, this gives a greater surface area for the microbes to do their work and also increases the air space in your pile which benefits the bacteria again since they are aerobic and need oxygen to survive , whole leaves will mat down and leave less air space.
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