South Carolina is a very special place. From the coast to the foothills of the Appalachian Mountains, South Carolina has a diversity of climates and landscapes. The diversity of climates allows for different grasses to flourish. Warm season grasses such as zoysia, St. Augustinegrass, bermudagrass, centipedegrass, and bahiagrass flourish at the coast throughout the year, but those grown in the upstate go dormant in the winter. In the dormant stage, the grass turns brown and looks dead, but new growth will appear in spring. Cool season grasses, such as ryegrass and certain fescues, grow best primarily in the upstate but go dormant, or do not survive the heat of summer. Here too, the grass looks dead, with regrowth appearing as the weather begins to turn cool in fall and flourish through spring. Dormant grass still has live roots in the ground that require water, just not as much as when they are actively growing. Unless it has been uncommonly dry or windy, natural rain events are enough to sustain dormant grasses.
Cool season annual ryegrass may be seeded into bermudagrass for a green lawn throughout the year. This is common throughout the state, and may be the reason why you see both green (actively growing grass) and brown (dormant grass) lawns right next to each other.
For more information on different types of lawn grasses grown in South Carolina see, HGIC 1214, Selecting a Lawn Grass and HGIC 1206, Overseeding With Ryegrass for information regarding overseeding with ryegrass.