Whole map and DLCs covered.

Weather mods with early winter nighttime at 5pm may or may not need adjustment for consistency. Nonetheless its fine.

The rainbows will show up in the morning on the western regions of Skyrim and in the afternoon on the eastern regions. Middle gets a compromise with more hours.
There is an option for the same daytime schedule regardless of the region they are in.
Dawnguard Solstheim get all daytime because lesser time is spent there.. Places like Markarth will be the same 
Feedback is the best thing you could give.
Depending on the weather the strenght and colors change. Most likely you don´t need the fainter textures. The colors are subdued pending the weather. The file is there nonetheless.

Standalone addons

Interiors addon - Some interiors have large waterfalls and/or groves with exterior light exposure. Rainbows in these conditions only.
                               Some  with custom made rainbow textures to mimic starlight - always present, no schedule. 
                               p.e. Blackreach, Eldergleam Sanctuary, etc
                                                    
Rainbows over Player Homes - A rainbow over your house.
                                                       Once you buy a certain decoration for a house, a chain, a lever or a button, an activator, will appear on the house walls                                                                         outside for you to toggle a rainbow hovering your new home. On/off feature. Off by default. 
                                                       Hearthfires homes not included. AE player homes not included. Always present; they do not follow a schedule.

Solitude Loompa Land - A sample. Very high on sugar. Maybe you want more? 

A rainbow over the Temple of Mara at the player wedding, if you want you can find it here - Better Skyrim Parties - Author Patches  

No requirements. Espfe´d, esl-esp.
This mod is subtle and pretty. and.. that´s about it. 
Rainy weathers custom activation things like that, I won´t do it.
Not interested in doing the same for the worldspace. There are limitations to the meshes. If you want to expand for bigger ones use the smoothie mesh.

You can expand this to other mods/worldpsaces if you want, p.e. Bruma

It is not impossible that some of the rainbows positioning is not perfect". Tell me how it is so if you want, and it will be addressed.

Compatibility ✅
Recommended use with an enb - no enb is fine too - the foam and spray particles in enbs are much more denser which make rainbows appear have more sense. Nonetheless, the mod is at its best using one.
Water Effects Brightness and Reflection Fix 
Water for Enb - recommended - contains big improvements for waterfalls; mist blasts, foams, smooth slopes, etc. Use with enb or not.
TMD Epic Waterfalls should be compatible by default
Landscape fixes mod - don´t know 
Elfx/shadows
Lux - patch provided
Fits a dark-themed modlist: fall for a brief period of time to a tranquil experience of colorful bliss.
Natural Waterfalls - patch provided
Nothing to patch.

The mod natural waterfalls is compatible. Although, it repositions a lot including water foams/sprays that are lessened..
Keep in mind Rainbows over waterfalls was not made with Natural Waterfalls in mind. There is a patch available for it, although there still may be adjustments to be made. p.e. rainbows do not show if behind foam, or flicker if intermeshing it. Or out of place, larger than they should, etc.

Initially I had tighter schedules but it was not worth it imo.

The scripts are very plain and very basic. They are active once a day in a split second only. To put this in perspective, if you pull a game lever once a day you get maybe the same script action as this. Maybe that´s not right. 

I have only tried this with vanilla weather and Azurite weather 2; no enb and pi-cho enb.

limitations and general information

Warning: do not try to optimize these meshes - don´t do it.