This premium feature allows flagging of attacks with a ball of various colors. Using it you are able to mark severe attacks in red (or another colour) for example. In addition, attacks marked with the green ball will not show incoming attack warning (red swords) in the village list.
You may have noticed the grey ball beside incoming attacks. If you click on this grey ball 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.