During the last Zombie Apocalypse, the US Government retreated the military west of the Rocky Mountains, abandoning the eastern two thirds of the continental United States, in order to establish a primary Safe Zone on the west coast.
Many pockets of survivors or even large enclaves still managed to survive, which were categorized as different color-coded "zones":
Safe Zones durinig World War Z, shaded blue (map is conjectural)
Green Zones[]
Green Zones were "active military facilities", such as large scale munitions plants which were too vital to abandon. A primary example of a Green Zone is the Rock Island munitions factory. Some were still actively contributing to the war effort, while others had no immediate strategic impact. Some of them also grew into refugee hubs, though this was not their primary purpose.
Red Zones[]
Areas that were deemed "Offensively Viable", consisting of factories, mines, power plants, etc. The army left behind custodial teams to defend and maintain these area for the time when they could be safely re-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.
Purple Zones[]
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[]
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 varied in size. Blue Zones ranged in size from sturdy houses and mansions, to towns and villages. Blue zones were rarely self-sustaining. They were 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, around the twin stadiums.
White Zones[]
White Zones were the zones totally overrun by the undead, which during the height of the war consisted of most of the planet's surface. While the common term after the war is "White Zones", on wartime maps these regions are actually shaded in light gray. It can be assumed that most humans in these areas had been killed, though small isolated pockets of survivors (not official "Blue Zones") and with no radio contact) could be found - along with other threats such as feral animals, feral humans, etc.
Major "White Zones" remaining even after the war are Iceland, Scandinavia, Siberia, the North American arctic, and much of the ocean floor - as well as major mountain ranges such as the Andes in South America, where zombies can freeze above the snowline only to thaw out later.