Changing ImageFormatSettings.color_mode

Hi,
I want to save my renders with an alpha channel automatically, but I don’t know how to proceed. I think it has something to do with the color_mode :
http://www.blender.org/documentation/blender_python_api_2_63_2/bpy.types.ImageFormatSettings.html?highlight=color_mode
Anyone knows how to change the ImageFormatSettings.color_mode to RGBA from a python script?
Thanks