Fallen leaves are browns in my book. I
should check the chemistry, but I feel like once the tree ceases photosynthesis and draws sugars down into the
roots for winter storage, the leaf has lost it's "greenness". The dead leaf is probably nothing more than a
carbon matrix of lignin and limited amount of nutrients and minerals such that only fungus, worms and soil microbes find them of any nutritive value. Leaf mold is a common way of composting them in any mass.
I use them as bedding in my chicken coop over winter and as mulch on my experimental food forest. I also use them in my indoor worm bin to add a little extra bulk when things get too moist.