It depends on how you want it to work and what you think is reasonable.
There is a difference between travel zones and actual interdiction.
So IMTU I do it like this.
1. Total System Wide Interdiction - the IN or the IISS will warn all ships jumping insystem to refuel and leave (refuel at designated points or else). Note that this level of interdiction is expensive, and coverage of all but the major worlds within the system is unlikely to be more than a sensor net and warning beacon. But ships are on the way, and they are authorised to shoot so its probably best to do as you are told.
This system will be red zoned by TAS.
2. Individual world Interdiction - the IN or IISS denies access to one or more worlds within the system, but travel is still allowed to other worlds/colonies/bases/stations exempt from the interdiction.
The interdicted worlds are red zoned, the system will get an amber.
3. System Red zone - due to reasons (xenophobia, the tendency to eat guests, take your pick) the TAS has declared the entire system a red zone - only travel with extreme caution, read up, and plan ahead. Good luck if you ignore the travel designation, but would you mind writing about your exploits if you get back safely?
4. World red zone - similar to the above but its only one or more worlds you need to worry about the carnivorous psionic plants. The rest of the system gets an Amber.
5. System Amber zone - similar to the above but they may not eat you, just give you lots of grief.
6. World amber zone - the rest of the system is green
A balkanised world/system could be an eclectic mix of travel zone designations...
