Currently, "Clear Color" of TextureRenderTarget2D has not been published to the editor. However, there is a request to control ClearColor to use RT as mask in UMG. So, I suggested that it should be published.
I tried to edit code of TextureRenderTarget2D.h
Before
/** the color the texture is cleared to */
UPROPERTY()
FLinearColor ClearColor;
After
/** the color the texture is cleared to */
UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = TextureRenderTarget2D, AssetRegistrySearchable)
FLinearColor ClearColor;
Result
[Image Removed]
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-55347 in the post.