I like your idea of modifying tree subset, I have a similar problem. I created a tropical island scene and used the ecosystem to foiliate the island. My problem is that the coconut trees have an un-natural brown color trunks. Having been to the tropics, and having reference pictures from there, the trunks are actually gray with slight variations of  light browns.  I was able to modify one palm tree to the color i like, but how do I change the color of the palm trunks in the eco-system I've already "planted"?. (I ask this question because I've got the trees just how I like them-overhanging the water, etc.) How do I add the new colored palm to the tree set? I've only had this software a couple weeks and I like it alot!  I'm including a small rendering, and you can see the edited tree 2nd from the right, I didn't scale it, so it's a bit large for the scene. I just wanted to get the color right. Ideally I'd like to modify/edit most of the trees to get more variation in eco-systems. Notice how all the "brown" trees look 'rubber stamped', the fronds are all different,cool, but the trunks are all carbon copies. Thanx for any light you may be able to shine on this subject, Ron