Now that the new leaves growing from your graft are receiving sun light and begin to transpire, enabling them to draw in more of the trees nutrients, they will darken just fine. Young leaves are a lighter shade of green than fully mature and aging leaves. The older leaves have their full complement of photosynthetic pigments and their cell wall maturity has become complete, both which contribute to the darker green tone. For new young citrus leaves, it normally requires a month or two's time to grow into a fully mature leaf, then they will join with their older brother leaves in the ranks of full coloration. Millet