Flash CS5.5 Professional.
symbolInstance.useBackgroundColor
Property; a boolean value that indicates whether to use 24 bit mode (true) or 32 bit mode with alpha (false) for the instance. If true, the backgroundColor specified for the instance is used.
The following example sets the useBackgroundColor property of an instance to true:
fl.getDocumentDOM().getTimeline().layers[0].frames[0].elements[0].useBackgroundColor = true