The V-Ray User Color node 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. Visit the User Attributes Workflow page for information and tutorials on user-defined properties. 

This texture also has the ability to read mesh color sets, UV sets, and extra channel information from VRayProxy objects.

User Attribute – Specifies the name of the user attribute that is looked up and returned from this shading node.

Default Color – Specifies the color to return, if the user attribute is not found on the shaded object.