Flash set background code for movie clips

Create a new Flash document named Background.fla.
Draw a blue circle on the stage.
Select the Painted blue circle and choose Modify > Convert to symbol.
Select the movie clips option, and then click OK.
Select an instance on the stage, and then type MY_MC in the Instance Name text box in the property inspector.
Select Frame 1th in the timeline, and then type the following code in the Actions panel:/* If Cacheasbitmap is set, the internal bitmap is opaque and can be rendered more quickly. */
My_mc.cacheasbitmap = true;
My_mc.opaquebackground = 0xff0000;

Select Control > Test movie to test the document.
The movie clip will appear on the stage with the background as the color you specified.