What red photography?

Using the red filter, the subjects in dark blue, medium gray, light green, orange, red and other colors are photographed as black and white films. After the spectrum corresponding to the above color information is filtered by a red filter, the obtained tones are black, dark gray, medium gray, light gray and light gray, with different gray levels.

If you take a color photo, the color will be magenta, crimson, yellow, light red and crimson through the red filter, and the photo is red as a whole.