Unfortunately it's not possible to use Forest Color separately from the scatter, because for certain features it's necessary to generate data structures that are stored in the Forest object.
One of these structures is the hierarchy of elements in the mesh. By technical reasons, these data cannot be generated by the map plugin.
If you want to test Forest Color in a single tree, you may create a Forest Object in Custom Edit mode, with an unique item.
The definition of an element in max is:
Forest doesn't use Max elements, but its own structure as described in the documentation
The Elements are the mesh pieces that define an Item, as the leaves or the branches of an individual tree. Note that the Elements are not the faces of a mesh, but an upper hierarchy. Internally, Forest Color consideres an Element as each group of adjacent faces of a mesh that share the same material ID.
We use this method because it works better to identify elements as leaves in a tree. Also, the hierarchy of max elements may change, depending of how faces were attached to compose the mesh. That would generate unpredictable results in Forest Color.