Reference:Radial Pattern

The radial pattern is a radial blend that wraps around the +y-axis. The color for value 0.0 starts at the +x-direction and wraps the color map around from east to west with 0.25 in the -z-direction, 0.5 in -x, 0.75 at +z and back to 1.0 at +x. Typically the pattern is used with a frequency modifier to create multiple bands that radiate from the y-axis. For example:

pigment {
  radial
  color_map {
    [0.5 Black]
    [0.5 White]
    }
  frequency 10
  }

creates 10 white bands and 10 black bands radiating from the y axis.

The radial pattern has a default color_map built in that results in a yellow, magenta and cyan pattern with smooth transitions.