Try revising the material properties of the texture for the leaves. Click on the material editor for leaves in the lower right corner of the tree editor. I think reducing the ambience setting (line 8 in the material lab) down will get more realistic shadws. Also make try setting the ambient color to the same color as the diffuse color. Or better yet, use the same texture for both ambient and diffuse light (the top two lines of the materials lab). This ought to help your grey shadow problems. My perception of this is that Bryce doesn't use a good physics model for ambient light/shadows. I keep hearing the term radiosity thrown around, as a better ambient light model, not that I know much more than this about the subject.