At the moment the color parameter is only valid for light sources. You can have a look at the API documentation, there is a description of the SceneNodeParams Enum that tells which params are valid for a node. Concerning the performance, the scene graph is still completely unoptimized. It doesn't have any dirty flags yet but that will change in the near future of course. The next release of the engine for example will already give you a very strong performance boost for rendering scenes with shadows
