Use 16 colors in 3D model representations

    out_dir = scatterer['out_file'].absolute().parent

    out_model_path = Path(str(geometry['out_file']) + ".obj")

    out_material_path = Path(str(geometry['out_file']) + ".mtl")

    # The following color scale is obtained from the normalized

    # RGB values of colors divided by 2.

    color_strings = [

        "0.5 0.5 0.5\n", # white

        "0.3 0.0 0.0\n", # red

        "0.0 0.0 0.3\n", # blue

        "0.0 0.3 0.0\n", # green

        "0.50 0.50 0.50\n", # white

        "0.30 0.00 0.00\n", # red

        "0.00 0.00 0.30\n", # blue

        "0.00 0.30 0.00\n", # green

        "0.35 0.35 0.00\n", # yellow

        "0.21 0.03 0.34\n", # purple

        "0.00 0.49 0.49\n", # cyan

        "0.27 0.16 0.08\n", # brown

        "0.19 0.21 0.09\n", # olive

        "0.50 0.00 0.50\n", # magenta

        "0.16 0.38 0.37\n", # turquoise

        "0.25 0.25 0.25\n", # gray

        "0.50 0.36 0.38\n", # pink

        "0.50 0.32 0.00\n", # orange

        "0.48 0.45 0.34\n", # vanilla

        "0.16 0.39 0.24\n", # emerald

    ]

    color_names = [

        "white", "red", "blue", "green"

        "white", "red", "blue", "green", "yellow", "purple", "cyan", "brown",

        "olive", "magenta", "turquoise", "gray", "pink", "orange", "vanilla",

        "emerald"

    ]

    mtl_file = open(str(out_material_path), "w")

    for mi in range(len(color_strings)):

                str_line = tmp_model_file.readline()

            else:

                old_sph_type_index = sph_type_index

                color_index = sph_type_index % (len(color_names) - 1)

                str_line = "g grp{0:04d}\n".format(sph_type_index)

                color_index = 1 + (sph_type_index - 1) % (len(color_names) - 1)

                str_line = "g grp{0:04d}\n".format(sph_type_index - 1)

                out_model_file.write(str_line)

                str_line = tmp_model_file.readline()

                str_line = "usemtl {0:s}\n".format(color_names[color_index])