To do this use a simple value which isn't attached to a field.
1. Add a simple value control to the page, size it to fill the page & remove the shadow/border (if required).
2. Set the normal background colour (e.g. transparent). Save/publish the page. Don't attach the simple value to a data source.
3. Go to the threshold rules & setup the rule making sure to set the background colour to change when it's triggered.
4. Save the rule, go back into the page and select the simple value, choose the threshold rule from the dropdown.
5. Finally go to the page display & refresh it.
Note: Using the simple value, you can set the "blinking" option as well which will make the background flash when it changes colour