The Alerts serve as an early warning or notification about active marches that are coming toward your base. Alerts for different activities differ by color as follows
Red Alert notifies you that your base or armies on the world map are being attacked by other players.
Attack - Alliance
Purple Alert notifies you the Buildings of your Alliance are being attacked by other players.
Yellow Alert notifies you that another player has sent a scout march toward your base.
Blue Alert notifies you that your allies are reinforcing your base with troops.
Green Alert notifies that resources are sent to your base.
The Alert buttons on your main command interface provide access to all current alerts. Alerts can be easily disabled via the "Ignore All" button on the main screen - this will disable the visual overlay for all alerts. All ignored alerts and their visual overlay can be re-enabled via the "Allow" button.
Additionally the Warfare and Alliance Spam Protection feature lets you disable the Alert Overlay and Push Notifications for attacks with specific total force (between 0 and 50K). For more details about Push Notifications check the Settings page.