Hey NORRIMO the description says that you should use a color palette similar to the gameboy which uses a 2-bit (4 shades of "gray" {light to very dark olive green}) see wikipedia. I guess adding black and white are two more colors. But on the other hand it also depends on the people participating in the jam and they rate it, so if they think black and white are additional colors they might let that influence the evaluation. Nonetheless gamejams are all about fun, so don't hassle to much, follow your idea and start jammin'!