Broken: I tested 2 recent versions and this was brought up on Blender Stack Exchange as a question.
Short description of error
The color input for shaders breaks the shader when used. Only the built-in color selector works for coloring the hair. Using a texture as input causes the hair to use default color(light-gray), using the attribute node for vertex colors breaks the shading for the hairs completely.
Exact steps for others to reproduce the error
I've prepared a .blend which has three node groups in the same material. You can just click each output node to see the results in the Rendered View.
Best guess: color input for shader nodes should be cast to the proper type. (hopefully that's not a red-herring) :)