They mean very little. It's really a case to convince the public:

a) That the government knows what they are doing. They do this in the full knowldge that terms like "Code Red" will impress the majority of people - It doesn't really tell them anything but it sure as hell sounds good!

b) That the government will be justified in reducing civil liberties such as freedom to protest against war as there is a "Code Red" or whatever situation.

Jon (a UK citizen)