LuxCoreRender Materials Mirror
The Mirror material represents a perfectly specular mirror.
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. 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 it being almost entirely water). You can vary the thickness to achieve the desired effect. A thickness of several hundred nm will result in the most pronounced effect.