You can use a color matrix filter. In this example, the original image is in the selected state, ie blue, and the color matrix filter changes it to solid white in the UP state. I'm sure there might be a better way, but this has worked for me.
<s:BitmapImage id="iconDisplay" horizontalCenter="0" verticalAlign="middle" verticalCenter="0">
<s:ColorMatrixFilter includeIn="up" matrix="[1,1,1,0,0,1,1,1,0,0,1,1,1,0,0,0,0,0,1,0]" />
Hope that helps.