Changing the lighting and the fog color does not have the desired effect. You need to replace most of the the textures as well. Have a look at the textures in Phil's Relto. You'll see that they are different than the "normal" Relto textures.
If you want to make it a day/night toggle you are going to need two versions of just about everything. Then you have to design a mechanism which toggles the layers based on an SDL value.
You could make different day and night objects and toggle those on/off. That would be the least difficult method but it makes the age almost twice as large.
Or you can keep the same amount of objects, add night layers to them and toggle those using opacity animations.
Or you can keep the same amount of objects with the same amount of layers but turn the layers into plLayerSDLAnimations. This would be the most elegant solution but it is also the hardest.
Probably not what you wanted to hear but if it was easy it would have been done long ago.