I wasn't suggesting you change the black and white colors - I meant make the "lack of a hint" dark-gray color less dark (but not too light, because then you'd have the opposite problem, with the white hints being too hard to see). I'd definitely keep white and black as the hint colors.
See attached for what I mean (top is what you have now, bottom is what I suggest (although hopefully you'll spend more than the 15 seconds I did on changing the color).
Or you could change the silver-colored background to some non-grayscale color (maybe a metalic pink similar to the text in the logo? or the gold you use for the play button?)... anything to make the black and white hints easier to see in comparison to the "lack of a hint" holes. I'm sure you could come up with something that looks good considering how nice everything else looks in the game.