White paper with three colors of writing: red, blue and green, photographed with a red filter

When shooting with a red filter, red writing on white paper can be photographed clearly, while blue and green writing will appear blurry. This is because the red filter only allows red light to pass through, so only red writing will be clearly photographed. The blue and green writing are absorbed or scattered by the red filter due to different wavelengths and cannot enter the camera lens, so they will appear blurred or disappear.