I really like the core of this game! I do think you could have conveyed the health and regen a bit more clearly, especially since they can get lost when there are lots of enemies on screen mobbing the player (even just a couple bars above the player's head could go a long way -- some folks have trouble with distinguishing different shades of the same color in high pressure situations like the ones your game throws at folks).
If you revisit this idea, I'd love to see larger levels with more obstacles and enemy-spawns (consider having distinct waves that the player can grow accustomed to -- Devil Daggers may provide some inspiration for you). It would also be cool to see the grass color locations change to introduce some additional desperation and wild movement in the player -- I think that would be super fun!
As it stands, you have a strong foundation for a solid game. I'd love to play it some more when you've had a chance to revisit it. Great job!