Flash MX 2004.
symbolInstance.colorAlphaAmount
Property; an integer that is part of the color transformation for the instance, specifying the Advanced Effect Alpha settings. This property is equivalent to using the Color >Advanced setting in the Property inspector and adjusting the controls on the right of the dialog box. This value either reduces or increases the tint and alpha values by a constant amount. This value is added to the current value. This property is most useful if used with symbolInstance.colorAlphaPercent. Allowable values are from -255 to 255.
The following example subtracts 100 from the alpha setting of the selected symbol instance:
fl.getDocumentDOM().selection[0].colorAlphaAmount = -100;