The alpha value of pixels can be set in transparent bitmaps only (i.e., bitmaps created with the value true passed to the transparent parameter of the BitmapData constructor)
package{ import flash.display.*; public class Main extends Sprite{ public function Main(){ var imgData:BitmapData = new BitmapData(20, 20, true, 0x330000FF); trace(imgData.getPixel32(0, 0)); // Displays: 855638271 // (Alpha is 0x33) } } }