Hello AJMW,
From what you described and your screenshots you have a Screen Capture 2D in your scene. That Screen Capture is returning a darker value than what you expect to see.
Scene Capture 2D Actors do not work like real reflections; they work more like a TV screen receiving an image from a camera. The Scene Capture 2D captures the scene from its view frustum, stores that view as an image, which is then used within a Material.
In the last screenshot you posted there is an area labeled Adjustments. I would try the brightness and vibrance in that section.
Please let me know if these troubleshoots work for you.
Thank you,