Hey Anwynn! Take a look at this :)
This is a common picture many RPG Makers have used in the past. It's an effect for making maps darker, more scarier, or dimly lit.
Second version
If this is too much, how about this one? It's a very similar effect to what I use in my game. As you can see it looks more foggy and adds detail in to the ground. It's my best effect. I can add a few tint variations for different types of locations to enhance them with a tint effect :)
Combined together. If you like both, this is how they would look together. It's just two pictures on screen with no scripting needed, and if you need help with getting them working I'm here to help.
Please let me know if you like them, and with your permission I'll e-mail the pictures you can use.