D
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File extensions are irrelevant. It's possible that there's a misunderstanding. It might be the bios colorization which you want to turn off. I'm not sure what Steve did with that, but perhaps turning off the "bios" option will do the trick.
I replaced the BIOS requirement with built-in color palettes. In my current build, GBC mode "Off" makes only GB games work in black in white, "If needed" makes GBC games work but GB games aren't colorized, and "On" makes GBC games work and colorizes GB games.