To access child objects you use the
This is a function from the Node object. So very node objects can use it.
What you have to do is:
var obj = obj_scene.instance()
obj.get_node("Sprite").set_opacity ( float opacity ) #for the opacity
obj.set_modulate ( Color modulate ) #for the color
because you only can acces children with the
get_node() function you have use the root "prefix":
to acces any node in your current scene. The advantage is that you can have nodes with the same name in a scene if they do have different parents.