The concentric circles of different colors mean different physical effects. The center of the circle is conflagration, where most people would die within 24 hours. The purple circle means 3rd degree burns, requiring medical care. The pink circle means 2nd degree burns, like burns from boiling water. And the yellow outermost circle means 1st degree burns, like a sunburn.
The map here shows the extent of damage if a nuclear device was dropped on Los Angeles.