There are many ways to achieve this, you could use Timer , Tween etc... here's a basic example.
var _count:int;
var red:Boolean = true;
var rectangle:Sprite = new Sprite();
var rectWidth:int = 300;
var rectHeight:int = 120;
addChild( rectangle );
addEventListener( Event.ENTER_FRAME , enterFrameListener );
function enterFrameListener(event:Event):void
{
if( _count > 0 && _count % 3 == 0 )
colorChange();
_count++;
}
function colorChange():void
{
var color:uint;
if( red )
color = 0x990000;
else
color = 0xfadd00;
with( rectangle.graphics )
{
clear();
beginFill(color);
drawRect( 0 , 0 , rectWidth , rectHeight );
endFill();
}
red = !red;
}