Good start on a space shooter!
I appreciate the variety of enemy types and enemy configurations.
In terms of the visual design, it would be good to have higher contrast--make the darkest color even darker.
It would also be nice to have some exciting music; you want to get the blood pumping in an action game like this.
Lastly, since we are looking at one-hit kills here, it would be good to have 3 lives--killing the player outright is a little harsh.