The V-Ray User Color 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 objects. The Example on the right uses V-RayUserColor to control the diffuse value on a single V-Ray Material.
UI Path: ||V-Ray Node Editor|| > Shader > Add > Texture Utilities > V-Ray User Color |