I remembered this fun little shmup a while back. It seems to be based off a PC-6001 game of the same name (which I think is homebrew on there as well) and it uses only 4 colors. The default color scheme is a little eye-searing but if you go to the second menu option and press Z you can change it to a slightly better one.
All the firing is done with one button and changes based on where you are. You can fire upward missiles on the ground and pressing down while on the ground and firing shoots off mortars. Holding fire shoots off a claw that can bounce back bullets and lasers and can heavily damage the boss (which is always a small core on a wall that fires off lasers)
Sometimes you get these bonus stages where you have to destroy UFOs and not the helis they're chasing (you see OH! MY GOD if that happens

I haven't quite figured out how to turn the mech around, the instructions say to tap down on the direction keys twice and it doesn't work. Sometimes it's happened by accident and I can't turn it back around (which is important as you have to use the claw to temporarily disable a laser blocking the way to the boss)
Worth checking this out if you want a fun little old style shmup.