The Thin Film material type produces an iridescent effect similar to a soap bubble.
The Refractive Index setting for Thin Film provides more or less reflection on the surface. Increase the value to get more reflection intensity. The actual colors you see in the thin film will be affected by the Refractive Index. However, you can shift the colors with the Thickness setting, so typically you will focus only on finding the desired amount of reflection with the Refractive Index setting.
Changing the Thickness setting will shift the colors that you see on the surface set as Thin Film. Increasing the setting to a very high value will result in the effect of layers of colored rings on the surface. It is generally a good idea to stay within a range of 100-1000.
The Color Filter setting acts as a color multiplier for the Thin Film material type. When the Color Filter is set to white the color of the material will be determined by the Thickness setting. Less saturated colors can be used to add subtle hue shifts while fully saturated colors have a greater influence. The Color Filter setting can be textured with color maps to create material breakups such as those seen on solar cells or to add surface imperfections to an optical lens or automotive headlight.