This game is interesting. I really like the color palette for each level, and the background art and promotional art is great. The main mechanic of it getting darker as the level goes on is a good start.
Sadly, it seems as the game goes on it slowly gets shoved to the side and becomes more of a nuisance instead of a challenge. I would have liked to see more mechanics adding on to or expanding it. The first few levels do very well with introducing the mechanic. As well as showing how the heart system works in conjunction with the visibility.
Also, the ghosts and the balls are too similar, I would recommend thinking about how the player deals with the enemies presented and if their approaches differ between them. I really like this game for it's art and music, but better game-play is needed.