The VRayOverrideMtl is a utility material provided with the V-Ray renderer. It allows a surface to look different depending on whether it is seen through reflections, refractions, or GI.
With this material you can get a fine control over the color bleeding, reflections, refractions, and shadows of the objects.
The example image by Linda Ferroni demonstrates the reflect material override option. Notice that the yellow bottle and brown pot are reflecting as entirely different materials in the mirror.
