Found it on a quiz worded this way. No other details unfortunately. I can only assume it has to do with the heated element materials of tungsten or molybdenum reaction with film layer materials. Perhaps it is a coating to reflect nothing but infrared. Hope to explain to students taking aviation maintenance course on ice and rain systems.
The heating element in cockpit windows is typically a thin film comprised of a transparent conducting oxide - indium tin oxide is common. In thin layers, it is transparent to the visible light spectrum but acts as a mirror in the infrared. As with all transparent conductive films, there will be some tradeoff between transparency and conductivity.
Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light. When the thickness of the film is an odd multiple of one quarter-wavelength of the light on it, the reflected waves from both surfaces interfere to cancel each other. Since the wave cannot be reflected, it is completely transmitted instead. Thus when white light, which consists of a range of wavelengths, is incident on the film, certain colors are intensified while others are attenuated. Thin-film interference explains the multiple colors seen in light reflected from soap bubbles and oil films on water. It is also the mechanism behind the action of antireflection coatings used on glasses and camera lenses.
For any certain thickness, the color will shift from a shorter to a longer wavelength as the angle changes from normal to oblique. This interference produces narrow reflection/transmission bandwidths, so the observed colors are rarely separate wavelengths, such as produced by a diffraction grating or prism, but a mixture of various wavelengths absent of others in the spectrum. Therefore, the colors observed are rarely those of the rainbow, but browns, golds, turquoises, teals, bright blues, purples, and magentas.
Additionally, keep in mind that in its thicker, bulk state, indium tin oxide is naturally yellowish-grey. Therefore, the thicker the film, the more it's natural color will influence the light passing through it. I don't know the natural colors for every transparent conductive film material, but it stands to reason that there will be some variation in natural coloration there as well, owing to the different elements included in the composite.