The green vs brown is to ensure that you have enough nitrogen (carbon/nitrogen ratio) for composting in your mix. C/N should be 24 to 1 for desired decomposition. Lack of nitrogen can slow decomposition. Green provides more N, getting you closer to the right ratio. The bacteria that do the decomposing require the nitrogen to do their work. Although it's possible to measure C and N to determine the ratio, I never do. If the pile is warm or hot, the bacteria have enough N.
Your autumn brown leaves will act as a brown ingredient. You can add green grass clippings, or manure, or fertilizer, or even "wee" to boost the nitrogen. Fortunately, the bacteria don't care where the N comes from!
