You may have noticed the grey ball beside incoming attacks. If you click on this grey ball (see the red arrow below) the colour of the ball will change.
The ball colour changes to green, if you click a second time it becomes yellow and with a third click it will be red:
If you want to have it grey again just click one more time on it.
This feature can be used as marker, it has no other functionality. Using it you are able to mark severe attacks in red (or another colour) for example.