I really liked the minimalistic colour pallet one thing about it though. Using some other colours for coins and the player (could be complementary could be a triad ex...) I use ColorHunt (https://colorhunt.co/) to help me find pallets that have variety while still maintaining the minimalistic feel of monotone pallets. Finally the difficulty. I would suggest respawning at the beginning of each level instead of all the way back at the start for the reason of its a game jam people typically won't want to spend nearly as long mastering mechanics and movement as you, the designer will have through testing.
Feel free to check out my game as well :)