alphaMultiplier (ColorTransform.alphaMultiplier プロパティ)

import flash.geom.ColorTransform;
import flash.geom.Transform;

var colorTrans:ColorTransform = new ColorTransform();
trace(colorTrans.alphaMultiplier); // 1

colorTrans.alphaMultiplier = 0.5; 
trace(colorTrans.alphaMultiplier); // 0.5

var rect:MovieClip = createRectangle(20, 80, 0x000000);
var trans:Transform = new Transform(rect);
trans.colorTransform = colorTrans;

function createRectangle(width:Number, height:Number, color:Number, scope:MovieClip):MovieClip {
    scope = (scope == undefined) ? this : scope;
    var depth:Number = scope.getNextHighestDepth();
    var mc:MovieClip = scope.createEmptyMovieClip("mc_" + depth, depth);
    mc.beginFill(color);
    mc.lineTo(0, height);
    mc.lineTo(width, height);
    mc.lineTo(width, 0);
    mc.lineTo(0, 0);
    return mc;
}