ActionScript 3.0 :: Stopping Function Started With Play Button

I need to stop a function, the function is started with a play button and I want it to stop by a stop button, then play then stop, etc. The changeColours function has a timer.start() at the end that triggers another function, which then restarts the changeColours function. These 2 functions keep doing this repeatedly after the play button and MouseEvent trigger the first function. I just want to be able to stop the changeColours function and then be able to start it again at any time.

ActionScript Code:
play_btn.addEventListener(MouseEvent.CLICK, playColours);
function playColours(e:MouseEvent):void {
if(!PLAYSTOP) {
changeColours();
PLAYSTOP = true;
}}
stop_btn.addEventListener(MouseEvent.CLICK, stopColours);
function stopColours(e:MouseEvent) {
//changeColours() needs to stop!!
}