Responding to the title: no.
To change saturation you have to act on the layers themselves.
No other way to affect them since whatver you put on top is - well - on top…
Re the rest, probably forget cesium is a thing - and switch off the internet when the game is running, see what happens.
The material though, should have an input parameter as a texture object - but it could also be hard coded (and probably is, I wouldn’t bother making it accessible via unreal material editor if I was the one creating cesium) within the USF.
As such, you’d need to learn some HLSL and mess with the .usf that the terrain uses to bump up the saturation of the map.
Only other idea there is to hack the image at the router level
If you filter all images that pass thorugh your router from a certain address and mess with them from the router itself like any beginning hacker would do…
For instance by flipping them vertically so everyone in your house can’t use the interwebs for memes…
You can get the saturation you want, record a take, change the router back, and - assuming all you need is the take, you are done…
Mind you, hacking the router is easier than learning HLSL and editing a .usf 
(Or you wouldn’t have script kiddos).