Call Imagemagick to generate a scheme.

def imagemagick(color_count, img, magick_command):
    """Call Imagemagick to generate a scheme."""
    flags = ["-resize", "25%", "-colors", str(color_count),
             "-unique-colors", "txt:-"]
    img += "[0]"

    return subprocess.check_output([*magick_command, img, *flags]).splitlines()