The device can send trap for NPM server.
, such that it shows green when down and red when up.
Is this a type or do you really Green when down and Red when up? If yes, I don't think that's possible. Red is the known colour for a device being down.
But otherwise you monitor the backup device as you would any other device.
It is possible with a bit of a database hack. See my suggestion for volume status in the discussion below. You should be able to set the status icon to Down.gif for a specific Node ID when the status is detected as Up.