That is what lies under the "states" button when you are in edit mode. Place the server variable that you want to use as the control in the label field. Click on the "Hide Value" check box then go into states and set the color and transparency you need for the off and on states. If your server variable is not of the "off, on" variety you can define a custom "local off" value that will act as the off state. This should do what you want.