Nice one! I ran it on my megadrive just now. initially I got confused all the time about which button corresponds to which color, but I got used to it by the 2nd level and pressed the correct button most of the time without thinking about it. I like the later levels with crossing paths making it not immediately obvious where to shoot.
Sorry for the shitty picture, I wanted to snap a picture of it running on real hardware but I was being chased by the colorful beasts at the same time :)