RetainerBox Effect Material Tooltip text as of Unreal 5.5:
The effect to optionally apply to the render target. We will set the texture sampler based on the name set in the @TextureParameter property.
If you want to adjust transparency of the final image, make sure you set Blend Mode to AlphaComposite (Pre-Multiplied Alpha) and make sure to multiply the alpha you’re apply across the surface to the color and the alpha of the render target, otherwise you won’t see the expected color.