LuxRender Materials Mirror
From LuxRender Wiki
The Mirror material represents a perfectly specular mirror. It's a handy shortcut for adding a mirror to your scene. It is simply shinymetal without the microfacet-polish coat.
This is the color of the mirror. Adjusting it will control the apparent "brightness" of the "mirror world." A value of around .55-.65 is a good starting point.
This allows you to define a film coating on top of the polish. This usually results in a rainbow colored reflection. It has two controls, a depth and IOR. The IOR should match that of your film material (Cleaning solution, which can leave such a film behind, has an IOR of ~1.3-1.35, due to being almost entirely water). You can vary the thickness to acheive the desired effect if you do not know the exact amount. Thickness of several hundred nm will result in the most pronounced effect.