Hi, Forest Color doesn't work with stand-alone objects, only with items within a Forest distribution.
If you want to apply it on solo trees, i suggest to create them with a Forest object in "Custom Edit" mode, which allows to edit items individually.
The reason for this limitation is that each tree model must be analyzed to get the hierarchy of elements (leaves and so).
For efficiency reasons, this data is stored in a common cache within Forest. It's not available for objects not assigned in Forest->Geometry.