The easiest way is to use a layer with red background color. Place it above other layers, untick "transparent" checkbox and add an effect like Multiply or Overlay or Burn - whatever you like best.
You can then turn this layer visibility on/off using a Timer or "Every X seconds" event.
Every 1 second
AlarmTriggered=true
..Layer "Alarm" is visible : Set layer "Alarm" invisible
..Else : Set layer "Alarm" visible