Specular color is the color of highlights on a shiny surface. The highlights are reflections of the lights that illuminate the surface. For a naturalistic effect, set the specular color to the same color as the key light source, or make it a high-value, low-saturation version of the diffuse color.

In Autodesk VIZ, you can set the specular color to match the diffuse color. This gives a matte effect, making the material appear less shiny.