This looks like a "feature" of the SolidGrowth generator for this species. To illustrate this, make the leaves 100% transparent so they disappear and load solid colours into the trunk and stem materials. You'll notice the colour of the stems closest to the trunk is desaturated to a greyish tone. It seems the colour of any leaf materials attached to these stems is also automatically desaturated. I agree the desaturation is too strong and brown would be a better colour than grey for dead leaves, but unfortunately I don't think we can change this aspect of this species' SolidGrowth algorithm.