Alert level
Appearance
Green Alert

EDF will not attack you in Green Alert. To start missions your alert level MUST be at green.
There is a cheat which you can toggle that aloud you to have no green alert at all time.
Yellow Alert

EDF will attack you but will not call for help.
Orange Alert
EDF will now call for help and gunships will now start to attack you.
Red Alert
EDF will now do whatever it takes to stop you.