There is a way to get maps darker in order to get a night effect. For this you can use shadows. I first saw that technique in one game Comprehensible made and also used it for some of my own maps. There are different shadow types available, so that it is possible to stack them and make your maps even darker. If you leave some “holes” in your shadows you even can get some cool light effects. And there are also shine and fire tiles you can use for making your light effects nicer.
However, as soon as you have placed shadows on your maps you won’t be able to change the map content below without moving or deleting the shadows. And for a real day / night cycle a timer event is surely more helpful than having to track every place where a teleport is programmed. So yeah, as Macmor stated, you can already have some amazing stuff for your RPG Playground games. Unfortunately, this often is very cumbersome and you rather might end up with managing things the engine actually should do for you. For that reason I decided to wait for better features before I continue with creating my games on RPG Playground.
Btw.: The RPG Makers provides a tint screen feature, which can make your maps darker. I suggested it for RPG Playground and deWiTTERS inserted a ticket at Trello:
https://trello.com/c/NkalaeHh/555-action-tint-the-screen
As more people vote for it as sooner we will have it.
The planned particle system might be also useful for beautiful night effects. You can find its feature card at Trello too:
https://trello.com/c/b4t2sxsp/26-particles-for-fires-smoke-%E2%AD%90