White light when it hits a prism or water in the sky is divided into its primary PARTS or colors. White is made up of EQUAL parts of Red, Green and Blue. These colors ADD together to make white. We call this RGB color and is what TV sets work with.
An object we see appears red to us because it absorbs blue and green and reflects back only RED.
A colored filter that is RED will absorb green and blue and let ONLY RED light get through to our film. It will thus make red things look like they were white. Things that were blue and green will look blacker or darker because they did not get through.
In the center photo on the left a yellow filter was used. It will reduce blue while only slightly reducing the red and green colors. A yellow filter is used to make colors appear near normal in shade on black and white film.
The bottom photo was taken using a RED filter. Notice how it made the red can appear so light in grey that the letters rubber cement don't show up ver well now. The blue was completely absorbed by the filter causing it to look black in the photo. This is a dramatic or CREATIVE effect when used like this.
The photographer would use a RED filter whenever they want to make red things look light grey or when they want blue things to look darker. Red is used to make clouds from a storm look nicer. The folds in a cloud are blue in color and photograph very low in grey in black and white. The red filter will make them darker and thus more dominant.