There are two possible approaches:
Server side
Change the style on your WMS server so that it generates and serves translucent polygons.
Exactly how this is done will vary depending on what WMS server you’re using.
Client side
In Unreal, customize the material so that it applies the overlay with an alpha mask. First, create a new Material Layer Blend that looks like this:
This is a copy of the MLB_CesiumAlphaBlendBaseColor blend included in Cesium for Unreal, except for the addition of the Multiply node. In the screenshot above, it’s multiplying by 0.5, meaning that the top overlay is blended with the bottom one at 50% translucency.
Next, create a new Material Instance and set its Parent to M_CesiumBaseMaterial:
Then, on the Layer Parameters tab, change the Blend Asset for Overlay1 (or whatever overlay you want to be translucent) to the new Material Layer Blend you created above.
Finally, apply this new material instance to your Tileset by setting the Cesium → Rendering → Material property. If you’re using Cesium World Terrain or another terrain tileset with a water mask, you’ll also need set the Water Material property (and that will cause the water effect to disappear… let me know if that’s a problem).
Kevin