ppmrainbow
Generate a rainbow.
TLDR
Generate a rainbow consisting of the specified colors
$ ppmrainbow [color1 color2 ...] > [path/to/output_file.ppm]
Specify the size of the output in pixels
$ ppmrainbow -width [width] -height [height] [color1 color2 ...] > [path/to/output_file.ppm]
End the rainbow with the last color specified, do not repeat the first color
$ ppmrainbow -norepeat [color1 color2 ...] > [path/to/output_file.ppm]