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 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.