This is the code I've used to achieve graduated symbols and varying colors. If there is a better or easier way that I've missed, please let me know. This code snippet is from when the generateRenderer task is executed and complete.
var renderer:ClassBreaksRenderer = event.renderer as ClassBreaksRenderer;
for (var i:int = 0; i < renderer.infos.length; i++) {
var breakSymbol:SimpleFillSymbol = SimpleFillSymbol(ClassBreakInfo(renderer.infos).symbol);
ClassBreakInfo(renderer.infos).symbol = new SimpleMarkerSymbol("circle",(i + 5) * 2,breakSymbol.color,1.0,0,0,0,new SimpleLineSymbol("solid",0xffffff,1.0,2));
}
Thanks!