Although nothing captures the Game Boy Soul better than a GB ROM, you probably should have forced the background’s palette on the sprites, since GBC renders those using three additional colours.
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that’s a weirdly specific issue you’ve mentioned - you’d get colors only if you played on the GBC, which is also the case for most GB games, and on the GBC you can also choose the palettes for these games at startup (the key combo). My priorities were a lot more focused on adding as much gameplay/content as I could (this is still a jam game) since I was so late XD
also, I checked a bit GBStudio, I only applied the Monochrome option and I’m not sure you can have a preferred GBC palette until you make it GB+GBC, which I didn’t want as I was scared some kind of glitch would make more than 4 colors popup (I tend to be unlucky myself soooo…)
Also, I don’t think that’s the main issue with the game TBH XD I guess each one has their priorities
also, I think I’ll make the next version in color (GBC)