I really like the sprites and basic idea of the game, but I agree with Jack that you could use a little something to create variation in the gameplay instead of just spamming space, perhaps a rhythm mechanic as he suggested, or perhaps being able to switch between attacks that only work on a specific colour of enemy (although I admit I'm not sure how to go about this with the single control), just something to make the player think a tiny bit.
But in general good work!