The original game boy could only show shades of green. It doesn't really show the theme that well. If you're are using game maker studio 2, you can grey scale all the sprites(there are many gray-scale converters online), make a semi transparent green sprite the size of the window, make an object for it, and place it over everything else. It will make everything a shade of green. Besides that, this is a really fun game. Would make a great mobile game too ;)