RTP water is an approximation, like everything else on screen.
That screen tone is much better. The real reason not to just darken the screen is that when it gets darker, our eyes can adapt...but when the screen gets darker, it's just darker compared to everything else around you. At night, the colors are turned more monotone because our eyes can only see colors when it's brighter; when it's dark enough, our eyes can't pick out the colors but can still pick out the general brightness level. You could make it better if you could apply a slight blur effect, but I don't think that's possible; the best I could think of is to do a script in VX that constantly takes a bitmap of the screen, blurs it, and puts it in a sprite in front of everything else, and that would probably be rather slow. True post-processing graphics effects would require lower level access to the graphics system.
Finally, the blue/purple tint works because we've been trained by movies to think it's an appropriate approximation. Movies are never actually filmed at night because the cameras need more light. Instead, they're filmed during the day and they have a filter applied during editing, much like how we make maps in RPG Maker. For as long as movies have had color, night scenes have always appeared slightly blue, so that's what we're used to on a screen. I can't say exactly whether we want night time to be blue otherwise, but I can definitely say that there's no way to get screens to show us what night time actually looks like unless it's a really dark screen at night.
If anyone's familiar with Mystery Science Theater 3000 (a show from over ten years ago that made fun of really bad movies), there was one movie (I think it was Werewolf) where it was really obvious that a night scene was filmed during the day because there were dramatic shadows visible and the sky was pretty bright. I think the sun might have even gotten in the shot at one point!