C8 would be correct.
Uff whith c12 all is ok but with c8 images are drawn with very bad colors!!
c8 produces 256 color output to your mbm file, c12 uses 4K colors, basically your source mbm, might be using different color table, thus the target looks different. You could try using default color table on your source and make sure it is 256 colors, then the target should look excatly the same.