Flash MX 2004.
symbolInstance.colorAlphaPercent
Property; an integer that specifies part of the color transformation for the instance. This property is equivalent to using the Color >Advanced setting in the instance Property inspector (the percentage controls on the left of the dialog box). This value changes the tint and alpha values to a specified percentage. Allowable values are from -100 to 100. See also symbolInstance.colorAlphaAmount.
The following example sets the colorAlphaPercent of the selected symbol instance to 80:
fl.getDocumentDOM().selection[0].colorAlphaPercent = 80;