During the last Zombie Apocalypse, the US Government created designations to areas of the country based on how secure the area was, and if it could provide a useful service.
Green Zones were some of the "military safe zones". They were where the military left behind teams of custodians to watch over "offensively viable" locations. Some of these were eventually turned into refugee camps.
Areas that a deemed "Offensively Viable" consisting of factories, mines, power plants, etc. These zones were patrolled by a custodial team who worked to defend and maintain the area for the time when they could be safely occupied and utilized. A secondary use of the teams was to prevent any catastrophic meltdown at any power plants which would have lead to not only the danger of nearby zombies becoming radioactive but also killing any survivors by either the explosion or the aftermath.
Resupply air fields for military planes flying the Continental Airlift, dropping supplies to the many survivor enclaves in the eastern United States. Described as "few and far between".
Blue Zones were the "islands of life in the sea of zombies". They were the places where civilians, not military, managed to fight off the zombies and create a "civilian safe zone" that vary in size. Blue Zones range in size from sturdy houses and mansions, to towns and villages. Blue zones are rarely self-sustaining. They are usually dependent on constant supply airdrops from their government or whatever remnants of it remain. A rather large Blue Zone was located in the ruins of Detroit.
White Zones were the zones totally overrun by the undead. It can be assumed that all or nearly all life in these areas has been killed by zombies. Some of these "White Zones" are Iceland, Finland most of West Africa, the Canadian arctic, and the much of the ocean floor.