The VRayUserColor texture can be used to read user-defined object properties from the current material and convert them to a color value. This allows you to use the same shading network on several different objects in the scene, but to control various aspects of the material through user-defined properties. This texture also has the ability to read mesh color sets, UV sets, and extra channel information from VRayProxy VRayProxy objects.
The Example on the right uses VRayUserColor to control the diffuse value on a single V-Ray Material.