(https://www.nyip.edu/images/cms/photo-articles/make-your-rainbows-standout.jpg)
Source:To enhance the rich colors of rainbows, consider using a circular polarizer. With this simple filter, the blue skies will pop, making the bow's colors stand out against the darker background. With the ability to dramatically alter a scene, the polarizer is one of the most useful accessories a photographer can carry. In addition to the increased saturation, a polarizer cuts the glare on non-metallic surfaces.
This makes it the perfect tool for wet leaves, lakes, and streams. Just note, a polarizer will reduce the amount of light entering your camera by 2 stops. To compensate, you'll need to compensate by adjusting your exposure. One simple way to do this is to raise the ISO.
Source:https://www.nyip.edu/photo-articles/archive/make-your-rainbows-standout