Unfortunately this issue is much more complex that it seems: in order to assign unique colors by item and element, Forest holds an internal cache with data about the objects used in the scene. For each object is necessary to store a diagram of its geometry, dependency of elements and much more.
So, since Forest Color has not this information for stand-alone objects, most of their features cannot be used out of Forest. We considered to support in this case only a limited set of features (as tint by Item), but would be necessary to get an unique ID by object using some common property (object's name, face ID...) as other plugins do.
This solution would add extra complexity to the plugin and interface, and we think is not worth the effort, since there is an easiest solution: just create a Custom-Edit Forest object and add your individual items to it. In this way you can use Forest Color to their full potential, besides other nice characteristics as the Points-cloud view (very useful for cars and other complex objects).