You can only have different inactive button images for inactive windows.
You could always just change your active and inactive button images to look like different colored glyphs and make the actual glyph images transparent.
The issue with this is that I wouldn't be able to have separate maximise and restore "glyphs" on the image... It would be the same for the middle button whether it was supposed to be the maximise or the restore glyph.
Is there any other way around this? :/