So the above solution works well for most purposes, but you'll find that some dark scenes in movies or anything filmed in a letterbox aspect ratio will trigger the signal detection and disable your lights.
Rather than checking for a black signal off to the side of the screen (0.87 - 0.98), set your signal detection to watch for a completely green signal in the center-ish. This worked to disable the lights when the rainbow bar was sent by the capture card, without killing the lights when I still want them!