My apologies if that was too vague. When transitioning from day to night and back to day there are many variables within the components that I listed above which effect the colour/lighting.
I recommend you first start by going through all of the settings in those above listed items to make a nice daytime look. Record all of the settings once you are happy with the daytime look. Then, do the same for a nighttime look and once you are happy with the look, record all of those settings.
Once you have a nice set of daytime settings and nighttime settings you can use a timeline to not only drive the rotation of your directional lights (sun and moon), you can add float channels to the timeline to use as an alpha to transition between those day/night variables which you previously recorded. You would of course want to store those previously recorded variables in a combination of local variables within your blueprint and parameter collections (I suggest you study those and familiarize yourself with them before diving in to the logic).
So in summary, make a daytime and a nighttime look you are happy with by adjusting all of the settings to your desire and make a note of them.
Then, convert those in to parameters within a parameter collection so that you can easily swap day to night.
Once you have those stored, make a timeline which drives the movement of your lights and set up the transition points using a float channel which will be the alpha for the lerp between the parameters.
If you are new to this I understand that’s a lot to take in but try to break it down in chunks and learn one thing at a time, experimenting along the way.