There are many ways to do that. An overlay color is the simplest and fastest way to do it but it's not super good looking like the graveyard keeper method. You can also only use shaders instead of a color overlay and get some more control over the effect. Also, you don't need to follow every step of this article, you could just use ambient light/spotlight/normals to get a pretty decent night effect. This software creates the normal texture for you https://azagaya.itch.io/laigter