That was the gbc rom, I tried the gb rom just now and with that the screen is entirely black, although the music and sound effects still play. I just downloaded them a couple days ago so they should be the current version.
Doing some more attempts at playing it and it gets wierder - most of the time sprites are black when still and yellow when moving, but occasionally on moving to a new floor, they will be dark grey when still instead, so at least I can see them. And dying and starting a new run tends to scramble all the colors of the sprites, background, and menus up as well! This flashcart does not seem to like this game.