In my opinion the whole "color alerts" not only let the citizens know that there might be a potential attack and security is "becoming more tight" it also intimidates and may even scare any "terrorist" that might have been planning an attack on the country. Scaring them off thinking that security is so tight, that they would be caught or unable to commence their attack without risk of the plan failing.
This not only buying time for the government to hopefully get a lead and catch this terrorist suspect, but increases the chance of a "delay" in a potential attack on the country. for those who don't understand this simple concept:
Metal Gear Solid. You want to sneak into that room over there but there's two guards, ok simple.
Uh oh, they saw you. Alert goes up to RED and men swarm your area. You hide and wait for YELLOW where theyre all searching, then its GREEN and they all left so security is at a all time low. So then you can go kill those two guards and not worry about 50 million of them swarming you and successfully reach that room