The background feature is used to assign a color to all rays that don't hit any object. This is done in the following way.
camera {
location <0, 0, -10>
look_at <0, 0, 0>
}
background { color rgb <0.2, 0.2, 0.3> }
sphere { 0, 1
pigment { color rgb <0.8, 0.5, 0.2> }
}
The background color will be visible if a sky sphere is used and if some translucency remains after all sky sphere pigment layers are processed.