Changing The Cursor With Coordinates?

I am trying to change the cursor with coordinates.

I have attached the cursor to the mouse, which is a Red Circle

what I want to happen is when it gets beyond the half way point on the screen for it to turn blue.

I can do it as a rollover but I need to make it to it with if statements and coordinates.

Below is the code I have used to attache the cursor to the mouse

circle_mc.onEnterFrame = function(){
this._x = _xmouse;
this._y =_ymouse;
Mouse.hide();
};