The W Coordinate node can access the OctaneRender® W Coordinate System, which generates values along the length of hair geometry. The W Coordinate node can be connected to the input of a Gradient to control the start and end color of hair strands.
NOTE: If a mesh contains curves, and "render curves" is enabled on the mesh item, the Modo plugin will generate Octane hair geometry with W Coordinates. See Strand Hair and Fur for details.