This looks like a green poplar leaf that has not quite finished turning yellow. Which is partly true! But the green area is actually what Charley Eiseman calls a "green island" near or around leafmines and galls made by a tiny moth. A brief explanation here: "some leaf miners can induce green-islands on yellow leaves in autumn, which are characterized by photosynthetically active green patches in otherwise senescing leaves. " This from a paper by F. Gutzwiller et al, published in 2015.