Using both Hookers Green and Sap Green I have added some very simple shadow work to the leaf area. This area is not the focal point, so I have to work hard to keep it simple and loose. Because I just love those details I have a really hard time not going in there and painting every stem and vein.
To strengthen the values in the shadow areas of leaves there are some nice tricks with color you can use. One, use a cool blue wash (either French Ultramarine or Pthalo or Prussian), keeping the wash wet so it remains transparent. Another way to deepen shadow areas is to use a color that is opposite on the color wheel. So, in the case of green leaves, adding a cool red, like Permanent Alizarin Crimson, will greatly intensify the values. I have also used VanDyke Brown for this type of application, but it is a very grainy color, so if you want to wash to look clean and clear, use Alizarin instead. Keep these washes wet and transparent to keep the most life in your colors. Pigments that are applied too thick will look dull and lifeless.