I need a Texture2D to use in the GetColor function (which is a self-written function), but the SceneCapture2D (from which I get the texture) only gives me a TextureRenderTarget2D and the cast to Texture2D fails apparently.
You cant simply cast from one to another because these two are inherited from completele different objects and blueprints do not seem to have any functionality for this. But in c++ you can archieve what you want with the following function
UTextureRenderTarget2D* AnyTarg; // This is already initialized... only for example
UTexture2D* Texture = AnyTarg->ConstructTexture2D(...);