Any artists/visual designers that haven't heard of additive blending yet? It's really useful for making things glow. Also, it doesn't overwrite what's behind it quite as much as normal blending (unless you hit white) so you can make new shapes by layering without having to fiddle as much with transparency. I recommend it for any visual aesthetic that needs things to light up in a dark atmosphere or dark area. Also, you can expect some way of trying it out in any image editor that isn't just paint or built-in to game maker studio (though I actually don't know if studio 2 lets you do so). Just remember to place a background color (preferably a really dark one) so that you can actually see what you're doing.
I used it for the entire background (which is randomly generated), for the river itself, and for the fireflies and their reflections. https://itch.io/jam/gmtk-2019/rate/460395