They technically do not. They use to sprites layered on top of each other. If you want to use more than 3 colors per sprite, for an extra challenge, you will have to overlap objects.
that's not really a challenge. you just draw two sprites instead of one. and nobody would even know if you just used as many colors as you wanted in one sprite
ill admit for me it would be trivial to stack many images to completely ignore what is essentially the only rule. The color tool I made could spit out multiple images to cover the entire 64 color nes pallet easy.
