Flash MX 2004.
symbolInstance.colorMode
Property; a string that specifies the color mode as identified in the symbol Property inspector Color pop-up menu. Acceptable values are "none", "brightness", "tint", "alpha", and "advanced".
The following example changes the colorMode property of the first element in the first frame of the first layer in the timeline to alpha:
fl.getDocumentDOM().getTimeline().layers[0].frames[0].elements[0].colorMode = "alpha";