Nice start. The leaves are appearing too dark because they lack the translucency of real leaves. Placing the same color, but darker, as your leaves into the translucency channel will help with this.

Also, unless you like doing long renders, you may want to try Ambient occlusion rather than full Global Illumination for objects like this - at least while you're testing them. GI is expensive time-wise when rendering plants with leaves, so many times, I'll create a light array instead to imitate GI.