Edit the fur description (either in the Channel Box or Attribute
Editor).
Make sure the fur has an Inclination
value greater than 0.
Right-click the Polar field,
and select Create New Texture. The Create Render
Node window appears.
Click the Ramp button.
In the Ramp Attributes section
change Interpolation to None.
Modify the ramp so that one side of the
part has a value of 0.5 (gray) and the other has a value of 1.0
(white), as in the following illustration. Position the part where
you want it. (In this example, the part is in the middle of the
surface.)