I had trouble removing the selected and down state colors for a spark list using IconItemRender. If you are making a mobile app and using IconItemRender (instead of ItemRenderer) there is no autoDrawBackground property.
I figured I'd drop it in here after figuring it out thanks to this page: http://www.sajeevkumar.com/2012/01/08/flex-4-6-list-mobile-iconitemrenderer-background-image/