This parameter is a multiplier to the Diffuse Color. 0.0 means no diffuse color while 1.0 is the full affect. The default is 0.0 which provides a very realistic dark to medium hair but with potentially higher rendering cost. You may need to use higher amounts for lighter hair like platinum blond or grey hair.
This is the diffuse color of the hair itself. Even for blond hair this is a darker shade than other color parameters for most realistic hair. For the best realism we recommend this be the same color as your Secondary Specular Color. The default is middle grey.
The bulk of your controls can be found under the Specular Parameters. Each of these relates in some way to the description below.