In Unreal's Material Layer system, can I create a Material Layer that adjusts the saturation of the layers below?

Cesium doesn’t use the Landscape system, genius. It uses the Material Layer system, not the Landscape Layer system. Which you would know if you’d actually read the link to the Cesium documentation that Ken posted in the first post. And it’s also absolutely trivially easy to set up layers in the Landscape Layer system that modify other layers, either individually or combined.