That's the biggest Achilles' heel of the Genesis/MD, too. The lack of color resolution. I helped make the palette more accurate for that MD port of Super Mario Bros, and I made two palettes - one to simulate the composite PPU palette, and one to simulate the RGB PPU palette. The composite palette was closer, and what Mairtrus ended up shipping with (the RGB palette, no matter which "side" of mid-values I chose, wasn't quite right due to the lack of color bits)