The command-line tools are provided by sumatrapdf-tool, which is installed next to SumatraPDF.exe. They only work after SumatraPDF has been installed.
Available in pre-release 3.7
Usage: sumatrapdf-tool recolor [options] <input filename>
A PDF can use several color spaces. You can use recolor to change the colorspace.
To convert a PDF to gray: sumatrapdf-tool recolor -o output.pdf -c gray input.pdf
The output colorspace (-c) can be gray (the default), rgb or cmyk. Use -r to also remove any output intents (embedded color profiles) from the file.

All options #

Usage: SumatraPDF recolor [options] <input filename>
  -c -    Output colorspace (gray(default), rgb, cmyk)
  -r      Remove OutputIntent(s)
  -o -    Output file