It's nothing special, you just have to decrease/increase each color in every palette by $10 in every frame until you reach $00 or $FF. The game is open source check out https://github.com/jrs0ul/nes_survival but it is pretty messy.
I do the same in my game but yours is abnormally smooth for the NES? I run your game in Mesen and its like the fade in/out does 256 shades of colors. I honestly don’t get it. Maybe Mesen is going something here..
