Thanks for the feedback :)
We've implemented a sound and the player changing color to red for a sec upon taking damage, aswell as the enemies turning red while they've been hit. This also works as a invulnarability indicator (while an objects color is changed they can't take damage).
This was our first project so far and we only worked on it for a day so we couldn't implement everything we imagined before we started sadly, because the game was full of physics related bugs that we focused on mostly at the beginning.