How to change the density and color of clouds, how to remove all shadows and maintain the same lighting?

Cesium for Unreal
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Using the sun sky provided by cesium

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I’m a little unclear about your question, because the CesiumSunSky doesn’t do any cloud rendering.

More generally though, CesiumSunSky is a fairly thin wrapper around Unreal’s built-in SunSky that adjusts it based on CesiumGeoreference parameters. So you might be able to find the answers you need in a more general Unreal forum.