Hmm, i'm not sure what you're after.. If it's global, then mess around with the lighting in WE (Some tilesets have more effect on the lighting than others -> you set use a custom lighting effect in the map options). Or just change the fog to black with a Z start of 0.
If you want to darken a single model at night I suggest a simple trigger: Event: Time of day becomes later than 18:00, Action: Set unit vertex colouring to 50/50/50% or sth like that.)