If you understand that filters block certain things and let other things through, you will discover that red filters will let red light through and block the other primary colors, green and blue. With this understanding, I thought to photograph plants in the backyard. I was interested in brown leaves, since brown is really a bad yellow and yellow light is made up or red and green. This means the red frequencies will go through the filter and record that part of the image on film. The green will be blocked, so green leaves will fall away into black or grey. Here is the result.