Make the RGB colors in a PPM image compatible with NTSC or PAL color systems. More information: https://netpbm.sourceforge.net/doc/ppmntsc.html.
ppmntsc path/to/input_file.ppm > path/to/output_file.ppm
ppmntsc --pal path/to/input_file.ppm > path/to/output_file.ppm
stderr
:ppmntsc --verbose path/to/input_file.ppm > path/to/output_file.ppm
ppmntsc --legalonly|illegalonly|correctedonly path/to/input_file.ppm > path/to/output_file.ppm