How to change the lighting?

I’ve been working with things like colormod, but it doesn’t make it darker, it just give everything a tint. I want to know, is there anyway to switch between night and day, or make it look like night?

Edit: I did find something that let me change it, which was good, but is there an ything out there that can help me change the sky?

Editing the map you use in Hammer.