By default Blender's Object Index passes are white only. This works fine for internal compositing but isn't very useful when you intend to do post work in an image editor (GIMP/PS).
This node group will add colours to your different pass indices (is that how you spell that?) and output the image to a file. Now when editing the final render in GIMP/PS, include this file as a layer to "select by color" the objects you'd like to isolate..