Why are the materials greyed-out on the tree object created with the editor?
The materials are automatically generated and therefore not directly editable.
Ok, so how do I get anything but white trees?
Materials are applied to branches/leaf groups in the “Geometry” tab for each one.
Basically what happens is you assign a separate material for each branch/leaf group and then Unity automatically atlases and generates materials for the entire tree!
super awesome 
Highlighting the trunk and putting a material on it changes the leaf material and visa versa.
Have you gone through the Tree Creator documentation yet?
I’ve looked in the install folder under documentation but found nothing mentioning trees except in the ScriptReference folder.
Wow, no, totally didn’t even think about that, thanks.