CONTRASTING FILTERS
Let's imagine, that before red, a green car stands against a brick wall. If the green color of the car and the red color of the wall have the same saturation, we get in a black and white photo uniform, almost a gray plane (the car will almost completely disappear against the background of the house).
To prevent similar phenomena, either a red lilter is used and the house will appear almost white in the photo, and the car – almost black, or a green filter, then the opposite phenomenon will occur.
The example above was extreme, but they are different, more everyday. You often see pictures of white houses against a black background, dangerous sky. They were obtained by applying an orange or light red filter, which darkened the sky and made everything stand out, what is not blue. So there were strong white clouds, white houses, yellowish field, and all in all gave an unusual effect.
The contrast filter can be in virtually any color and serves a specific purpose: highlighting a certain group of colors, while suppressing others (there are e.g. blue filters, to highlight the blue, and the muting of other colors). These filters do not restore the natural color balance, but introduce a completely different scale of their brightness.