Space Hippies

If you remember Space Invaders you'll probably know what to expect in this game, with enemy waves moving in a particular pattern which need to be defeated before they reach your ship. In this case the aliens are green groovie ghoulie ghosties or technicolor musrooms or giant glowing heads and each new level has a great multi-color game name splash. This one was never released in 1992, but was released to the scene with source code intact in 2017.