What’s happening here is that your camera is seeing a big white area (the back of the photo) and trying to compensate by making the exposure darker. If you shoot a lot of photo backs, then you might consider shooting in manual mode instead of aperture mode as I suggest in my course.
You could use a color checker, but what you need help with is exposure, not color balance. In that case, you can actually use the gray base of your copy stand for the same purpose. Adjust the exposure of these “photo back” shots so the gray of the copy stand base matches the gray of a “photo front” shot and you should be set.